QCalendarWidget: fix a missing break statement

GCC 7 warns about implicit fall-throughs, and here it looks
like a break was indeed missing. It surely isn't catastrophic
that the other update code is executed, too, but it's also
useless.

Turns out Coverity knew it all along...

Coverity-Id: 11162
Change-Id: I88fc0174a66ec337b2d93c006e70be8d5f3bbc33
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/widgets/widgets/qcalendarwidget.cpp

@@ -2994,6 +2994,7 @@ bool QCalendarWidget::event(QEvent *event)
     switch (event->type()) {
         case QEvent::LayoutDirectionChange:
             d->updateButtonIcons();
+            break;
         case QEvent::LocaleChange:
             d->m_model->setFirstColumnDay(locale().firstDayOfWeek());
             d->cachedSizeHint = QSize();