Fix #13083 segfault in `SDL_RemoveTrayEntry()` for submenu entries

Use `g_object_ref_sink()` in `SDL_CreateTraySubmenu()` as introduced with
3be67ced646f9d884c32ce6858f39fe9dd8d634b for the top-level menu.

src/tray/unix/SDL_tray.c

@@ -541,7 +541,7 @@ SDL_TrayMenu *SDL_CreateTraySubmenu(SDL_TrayEntry *entry)
         return NULL;
     }
 
-    entry->submenu->menu = (GtkMenuShell *)gtk_menu_new();
+    entry->submenu->menu = g_object_ref_sink(gtk_menu_new());
     entry->submenu->parent_tray = NULL;
     entry->submenu->parent_entry = entry;
     entry->submenu->nEntries = 0;