fix WIN32/ANDROID defined usage in core.h

[cscd98]

Fixes compile under MINGW. The use of #elif WIN32 is valid under MSVC but not MINGW.

../../core.h:36:12: error: #elif with no expression
   36 | #elif WIN32
      |            ^
In file included from ../../core.h:812:

I believe the same applies to android.