Allow including SDL_uikit_main.c in Apple multi-platform builds

This file just won't do anything on macOS

src/main/uikit/SDL_uikit_main.c

@@ -5,6 +5,8 @@
 /* Include the SDL main definition header */
 #include "SDL_main.h"
 
+#if defined(__IPHONEOS__) || defined(__TVOS__)
+
 #ifndef SDL_MAIN_HANDLED
 #ifdef main
 #undef main
@@ -17,4 +19,6 @@ main(int argc, char *argv[])
 }
 #endif /* !SDL_MAIN_HANDLED */
 
+#endif /* __IPHONEOS__ || __TVOS__ */
+
 /* vi: set ts=4 sw=4 expandtab: */