Apply WIN32_LEAN_AND_MEAN redefine fix to SDL_syswm.h too.

This is related to 36ffc47b6033, which fixes bug 2508.

include/SDL_syswm.h

@@ -52,7 +52,9 @@ struct SDL_SysWMinfo;
 #else
 
 #if defined(SDL_VIDEO_DRIVER_WINDOWS)
+#ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN
+#endif
 #include <windows.h>
 #endif