<wchar.h>: Explicitly add va_list definition.

<wchar.h>: Explicitly add va_list definition.

Looks like there are serious namespace pollution going on in wchar.h.
Still this header should provide va_list definition as per POSIX and not
rely on pollution that once gets fixed the defintion would be still there.

While there, add empty #ifdef va_start block to prevent dports gcc compilers
from "fixing" this header and causing header shadowing (current variant is fine).