Added support for building SDL as a dynamic library on iOS

src/core/winrt/SDL_winrtapp_common.cpp

@@ -29,7 +29,7 @@
 int (*WINRT_SDLAppEntryPoint)(int, char **) = NULL;
 
 extern "C" DECLSPEC int
-SDL_WinRTRunApp(int (*mainFunction)(int, char **), void * xamlBackgroundPanel)
+SDL_WinRTRunApp(SDL_main_func mainFunction, void * xamlBackgroundPanel)
 {
     if (xamlBackgroundPanel) {
         return SDL_WinRTInitXAMLApp(mainFunction, xamlBackgroundPanel);
@@ -63,4 +63,4 @@ SDL_WinRTGetDeviceFamily()
 #endif
 
     return SDL_WINRT_DEVICEFAMILY_UNKNOWN;
-}
+}