Fixed building using MinGW

Our SDL_windows.h needed to be included before anything else so UNICODE is defined.

src/atomic/SDL_spinlock.c

@@ -20,14 +20,14 @@
 */
 #include "SDL_config.h"
 
+#ifdef __WIN32__
+#include "../core/windows/SDL_windows.h"
+#endif
+
 #include "SDL_atomic.h"
 #include "SDL_mutex.h"
 #include "SDL_timer.h"
 
-/* Don't do the check for Visual Studio 2005, it's safe here */
-#ifdef __WIN32__
-#include "../core/windows/SDL_windows.h"
-#endif
 
 /* This function is where all the magic happens... */
 SDL_bool