don't assume windows gaming input available for mingw

[rohlem]

In updating dvui to zig version 0.14.0-dev.2628+5b5c60f43, on Windows I encountered the following build error:

...\zig\p\1220c3a9f874bda11563c0f1e7fa5bfab0f7378af40310dcd49426e7d1afe9483fb6\src/joystick/windows/SDL_windows_gaming_input.c:35:10: error: 'windows.gaming.input.h' file not found
#include "windows.gaming.input.h"
         ^~~~~~~~~~~~~~~~~~~~~~~~~
...\zig\p\1220c3a9f874bda11563c0f1e7fa5bfab0f7378af40310dcd49426e7d1afe9483fb6\src/joystick/windows/SDL_rawinputjoystick.c:67:10: error: 'windows.gaming.input.h' file not found
#include "windows.gaming.input.h"
         ^~~~~~~~~~~~~~~~~~~~~~~~~

I don't have MSVC installed, and I guess zig compiling via mingw headers doesn't provide this header.
Therefore to make the build succeed, I had to adjust this predefined macro not to assume the header is present.

Having build.zig options/integration for this would probably be preferable, however,
for whatever reason for .windows os tag the pregenerated config is chosen,
and I didn't want to touch any of that logic without the necessary context,
so this seemed like the minimal fix.

As far as I understand, __MINGW32__ shouldn't be defined when compiling for msvc ABI (which I assume includes the header),
so behavior should be the same for that target.