QHeaderView - call invalidateCachedSizeHint() on clear

This calls invalidateCachedSizeHint on clear, which is a logical
thing to do.

Task-number: QTBUG-22528
Change-Id: I0befb2d492599fa8a05b1c2162bbca586e1b019d
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

src/widgets/itemviews/qheaderview.cpp

@@ -3230,6 +3230,7 @@ void QHeaderViewPrivate::clear()
     sectionHidden.clear();
     hiddenSectionSize.clear();
     sectionItems.clear();
+    invalidateCachedSizeHint();
     }
 }