QMessageBox: Opt out of native dialog if standard buttons have menu

Amends 2d59f2e8ca, which only checked
this for custom buttons, but standard buttons can also be customized.

Pick-to: 6.5 6.6
Change-Id: Ie8bafcf5007a0b4789b2d0ee1ddbc660a952fee4
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/dialogs/qmessagebox.cpp

@@ -2824,7 +2824,7 @@ bool QMessageBoxPrivate::canBeNativeDialog() const
     if (strcmp(QMessageBox::staticMetaObject.className(), q->metaObject()->className()) != 0)
         return false;
 
-    for (auto *customButton : customButtonList) {
+    for (auto *customButton : buttonBox->buttons()) {
         if (QPushButton *pushButton = qobject_cast<QPushButton *>(customButton)) {
             // We can't support buttons with menus in native dialogs (yet)
             if (pushButton->menu())