Fixed a warning when SDL_syswm.h is included in code compiled for iOS with clang.

include/SDL_syswm.h

@@ -163,6 +163,7 @@ struct SDL_SysWMmsg
 #if defined(SDL_VIDEO_DRIVER_UIKIT)
         struct
         {
+            int dummy;
             /* No UIKit window events yet */
         } uikit;
 #endif