main: Fixed compiler warning on Android.

(the NDK got upset about a function with void params using a bare `()`.)

src/main/SDL_main_callbacks.h

@@ -22,7 +22,7 @@
 #ifndef SDL_main_callbacks_h_
 #define SDL_main_callbacks_h_
 
-SDL_bool SDL_HasMainCallbacks();
+SDL_bool SDL_HasMainCallbacks(void);
 int SDL_InitMainCallbacks(int argc, char *argv[], SDL_AppInit_func appinit, SDL_AppIterate_func _appiter, SDL_AppEvent_func _appevent, SDL_AppQuit_func _appquit);
 int SDL_IterateMainCallbacks(SDL_bool pump_events);
 void SDL_QuitMainCallbacks(void);