st/nine: Clean up WINAPI definition

st/nine: Clean up WINAPI definition

As Emil pointed out, only gcc, clang and MSVC compatibility is required.
Hence the check for GNUC can be skipped, as __i386__ and __x86_64__ are
only defined for gcc/clang, not for MSVC.

Remove the #undef which has been there for historic reasons, when wine
dlls for nine have been built inside mesa. Instead use #ifndef in order
to avoid redefining WINAPI from MSVC's headers.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Acked-by: Axel Davy <axel.davy@ens.fr>