wchar: Define va_list for POSIX (BZ #30035)

This was uncovered by a recent clang change [1].  Different than ISO C,
POSIX states that va_list should be exported by wchar.h [2].

Checked on x86_64-linux-gnu and aarch64-linux-gnu.

[1] https://reviews.llvm.org/D137268
[2] https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/basedefs/wchar.h.html