QFileDialog: add missing break statements in switch in labelText()

It is of course wrong to potentially return the text of the Cancel
button if the text of the Accept role button was asked for.

Found independently by GCC 7 and Coverity.

Coverity-Id: 11150
Change-Id: Ie30f7875daee16a78eeff4b314ce17cbd7cd3aa8
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/widgets/dialogs/qfiledialog.cpp

@@ -2042,10 +2042,12 @@ QString QFileDialog::labelText(DialogLabel label) const
             button = d->qFileDialogUi->buttonBox->button(QDialogButtonBox::Save);
         if (button)
             return button->text();
+        break;
     case Reject:
         button = d->qFileDialogUi->buttonBox->button(QDialogButtonBox::Cancel);
         if (button)
             return button->text();
+        break;
     }
     return QString();
 }