wcmUSB: fix undefined behavior for bit-shifting

Automated test runs have detected the following issue while running UBSan
checks:

~~~
../src/wcmUSB.c:1372:11: runtime error: left shift of 1 by 31 places cannot
 be represented in type 'int'
   	#0 0x7f0444bcbd8c in mod_buttons ../src/wcmUSB.c:1372
   	#1 0x7f0444bd7f26 in test_mod_buttons ../src/wcmUSB.c:2090
   	#2 0x7f0444bfcea7 in wcm_run_tests ../test/wacom-test-suite.c:46
   	#3 0x56204d77b405 in main ../test/wacom-tests.c:44
   	#4 0x7f0448625082 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x24082)
   	#5 0x56204d77b1cd in _start (/home/runner/work/xf86-input-wacom/xf86-input-wacom/builddir/wacom-tests+0x11cd)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../src/wcmUSB.c:1372:11 in

The default type of a literal `1` is `int` and thus a leftshift of 31 is
not permitted. Switch it to `1u` and thus `unsigned int` for Button 31
which is the max button we support (if zero-indexed).

Link: https://github.com/linuxwacom/xf86-input-wacom/actions/runs/7049012015/job/19186502078
Signed-off-by: Peter Hutterer <[email protected]>

src/wcmUSB.c

@@ -1369,7 +1369,7 @@ mod_buttons(WacomCommonPtr common, unsigned int buttons, unsigned int btn, Bool
 		return buttons;
 	}
 
-	mask = 1 << btn;
+	mask = 1u << btn;
 
 	if (state)
 		buttons |= mask;