tray: fixed multi-threading issues with GTk implementation

GTK+ documentation states that all GDK and GTK+ calls should be made from the main thread.

Fixes https://github.com/libsdl-org/SDL/issues/11984

src/tray/windows/SDL_tray.c

@@ -209,6 +209,10 @@ static HICON load_default_icon()
     return LoadIcon(NULL, IDI_APPLICATION);
 }
 
+void SDL_UpdateTrays(void)
+{
+}
+
 SDL_Tray *SDL_CreateTray(SDL_Surface *icon, const char *tooltip)
 {
     if (!SDL_IsMainThread()) {