FieldFuncOption to Ignore struct fields

[alee792]

When building a schema against generated protobuf structs, the following error is encountered:
panic: bad method potatoes on type schemabuilder.query: bad field xXX_NoUnkeyedLiteral on type pb.potato: bad type struct {}: should have a name

Since the field is generated, it can't be struct tagged. An IgnoreField FieldFuncOption would be great!