Fix timespec/timeval warning when compiling C extension modules

[tekknolagi]

On some compilers, the warnings below get generated. I think it's
something to do with not defining _POSIX_C_SOURCE when we include
time.h but that didn't work--so it might be a project-level setting
that we need to tweak.

In the meantime, forward declare the structs in cpython-types.h.

In file included from /home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-data.h:5,
                 from /home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/Python.h:15,
                 from signature.c:1:
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:1311:58: warning: ‘struct timespec’ declared inside parameter list will not be visible outside of this definition or declaration
 1311 | PyAPI_FUNC_DECL(int _PyTime_AsTimespec(_PyTime_t, struct timespec*));
      |                                                          ^~~~~~~~
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:21:70: note: in definition of macro ‘PyAPI_FUNC_DECL’
   21 | #define PyAPI_FUNC_DECL(DECL) __attribute__((visibility("default"))) DECL
      |                                                                      ^~~~
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:1312:57: warning: ‘struct timeval’ declared inside parameter list will not be visible outside of this definition or declaration
 1312 | PyAPI_FUNC_DECL(int _PyTime_AsTimeval(_PyTime_t, struct timeval*,
      |                                                         ^~~~~~~
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:21:70: note: in definition of macro ‘PyAPI_FUNC_DECL’
   21 | #define PyAPI_FUNC_DECL(DECL) __attribute__((visibility("default"))) DECL
      |                                                                      ^~~~
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:1316:65: warning: ‘struct timeval’ declared inside parameter list will not be visible outside of this definition or declaration
 1316 | PyAPI_FUNC_DECL(int _PyTime_AsTimeval_noraise(_PyTime_t, struct timeval*,
      |                                                                 ^~~~~~~
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:21:70: note: in definition of macro ‘PyAPI_FUNC_DECL’
   21 | #define PyAPI_FUNC_DECL(DECL) __attribute__((visibility("default"))) DECL
      |                                                                      ^~~~
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:1325:64: warning: ‘struct timespec’ declared inside parameter list will not be visible outside of this definition or declaration
 1325 | PyAPI_FUNC_DECL(int _PyTime_FromTimespec(_PyTime_t* tp, struct timespec* ts));
      |                                                                ^~~~~~~~
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:21:70: note: in definition of macro ‘PyAPI_FUNC_DECL’
   21 | #define PyAPI_FUNC_DECL(DECL) __attribute__((visibility("default"))) DECL
      |                                                                      ^~~~
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:1326:63: warning: ‘struct timeval’ declared inside parameter list will not be visible outside of this definition or declaration
 1326 | PyAPI_FUNC_DECL(int _PyTime_FromTimeval(_PyTime_t* tp, struct timeval* tv));
      |                                                               ^~~~~~~
/home/max/Documents/code/skybison/build-relwithdebinfo/include/skybison3.8/cpython-func.h:21:70: note: in definition of macro ‘PyAPI_FUNC_DECL’
   21 | #define PyAPI_FUNC_DECL(DECL) __attribute__((visibility("default"))) DECL
      |                                                                      ^~~~

[github-actions]

{
  "django_minimal_requests": {
    "benchmark": "django_minimal_requests",
    "cg_instructions before": 682502,
    "cg_instructions now": 683318,
    "cg_instructions ∆": "0.1%",
    "interpreter_args": [],
    "interpreter_name": "pyro",
    "version before": "ba3fe617c745c2f297df1b98e90d1020ccbb2eb5",
    "version now": "32960c735767d33f078d4538cc5a4235e52dd6c3"
  }
}

[github-actions]

Summary

Metric	Average	Best	Worst	Notes
cg_instructions	0.0%	bench_compile -0.0%	loadproperty 0.0%	typically < 0.2% noise

Benchmark details

Base vs. New

benchmark	cg_instructions
2to3	-0.0%
bench_base64	0.0%
bench_compile	-0.0%
bench_pickle	0.0%
deltablue	0.0%
fannkuch	0.0%
go	0.0%
loadproperty	0.0%
nbody	0.0%
nqueens	0.0%
pyflate	0.0%
pystone	0.0%
richards	0.0%

CPython vs New

benchmark	cg_instructions
2to3	-10.8%
bench_base64	-40.2%
bench_compile	1076.0%
bench_pickle	-28.0%
deltablue	-65.3%
fannkuch	-3.4%
go	-64.6%
loadproperty	-75.5%
nbody	20.7%
nqueens	25.5%
pyflate	-33.8%
pystone	-73.9%
richards	-80.1%

Base

benchmark	cg_instructions
2to3	2,287,551,572
bench_base64	2,882,313,099
bench_compile	2,905,491,131
bench_pickle	2,985,833,105
deltablue	1,430,221,175
fannkuch	5,397,359,841
go	1,864,328,484
loadproperty	427,313,438
nbody	9,408,474,760
nqueens	2,776,591,010
pyflate	9,583,576,867
pystone	1,038,574,892
richards	957,256,952

New

benchmark	cg_instructions
2to3	2,287,515,749
bench_base64	2,882,333,670
bench_compile	2,905,417,821
bench_pickle	2,985,881,979
deltablue	1,430,234,330
fannkuch	5,397,370,561
go	1,864,480,531
loadproperty	427,364,704
nbody	9,408,480,829
nqueens	2,776,598,029
pyflate	9,583,581,271
pystone	1,038,577,078
richards	957,258,955

CPython

benchmark	cg_instructions
2to3	2,564,111,609
bench_base64	4,819,171,422
bench_compile	247,069,506
bench_pickle	4,145,157,377
deltablue	4,118,645,788
fannkuch	5,587,461,051
go	5,263,097,076
loadproperty	1,747,800,158
nbody	7,793,866,772
nqueens	2,213,294,569
pyflate	14,472,097,572
pystone	3,981,820,336
richards	4,819,791,013