Always enable D3D12 GPU on Windows

This isn't dependent on the render subsystem

Fixes https://github.com/libsdl-org/SDL/issues/12922

CMakeLists.txt

@@ -3023,7 +3023,7 @@ if(SDL_GPU)
     set(SDL_GPU_D3D11 1)
     set(HAVE_SDL_GPU TRUE)
   endif()
-  if(SDL_RENDER_D3D12)
+  if(WINDOWS)
     sdl_glob_sources("${SDL3_SOURCE_DIR}/src/gpu/d3d12/*.c")
     set(SDL_GPU_D3D12 1)
     set(HAVE_SDL_GPU TRUE)