Define WINAPI_PARTITION_SYSTEM for older Windows headers

Define WINAPI_PARTITION_SYSTEM for older Windows headers

Some older Windows and MING64 environments does not have
WINAPI_PARTITION_SYSTEM defined.

This breaks build with strict warnings.

Add empty definition if this happens.

Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
Reviewed-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Norbert Pocs <norbertp@openssl.org>
MergeDate: Thu Apr 30 11:42:23 2026
(Merged from https://github.com/openssl/openssl/pull/30941)