Fix build on Windows targets without dxgi.h, like MingW32.

include/SDL_config_windows.h

@@ -76,6 +76,8 @@ typedef unsigned int uintptr_t;
 # define SIZEOF_VOIDP 4
 #endif
 
+#define HAVE_DXGI_H 1
+
 /* This is disabled by default to avoid C runtime dependencies and manifest requirements */
 #ifdef HAVE_LIBC
 /* Useful headers */