Don't call SDL_Gtk_Quit() on private platforms

(cherry picked from commit c742038aa96c53d90bf47c0974db1d20c9796390)

src/SDL.c

@@ -689,7 +689,7 @@ void SDL_Quit(void)
     SDL_DBus_Quit();
 #endif
 
-#if defined(SDL_PLATFORM_UNIX) && !defined(SDL_PLATFORM_ANDROID) && !defined(SDL_PLATFORM_EMSCRIPTEN)
+#if defined(SDL_PLATFORM_UNIX) && !defined(SDL_PLATFORM_ANDROID) && !defined(SDL_PLATFORM_EMSCRIPTEN) && !defined(SDL_PLATFORM_PRIVATE)
     SDL_Gtk_Quit();
 #endif