QMenuBar: honor the left widget size hint

expandedTo() returns the expanded size, so we must assign the
return value (otherwise it's a no op). We were accidentally
discarding it.

Task-number: QTBUG-36010
Change-Id: Ic70c12648382a6b2ef7d70809bf25caa4cbe2f3a
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/widgets/widgets/qmenubar.cpp

@@ -1632,7 +1632,7 @@ QSize QMenuBar::sizeHint() const
     if(d->leftWidget) {
         QSize sz = d->leftWidget->sizeHint();
         sz.rheight() += margin;
-        ret.expandedTo(sz);
+        ret = ret.expandedTo(sz);
     }
     if(d->rightWidget) {
         QSize sz = d->rightWidget->sizeHint();