vulkan/util: Handle enums that are in platform-specific headers.

vulkan/util: Handle enums  that are in platform-specific headers.

VkFullScreenExclusiveEXT comes from the win32 header. Mostly took
the logic from the entrypoint scripts:

1) If there is an ext that has it in the requires and has a platform,
   take the guard for that platform.
2) Otherwise assume it is from the core headers.

Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>