QTableView: fix cornerWidget connection

A click on the corner widget should select all items, not reset the
selection. This was accidentally added with
3e144bdc74.

Pick-to: 6.7
Fixes: QTBUG-124267
Change-Id: Ie20b7cf0ff730214dca80116ad888f42c2b7a670
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

src/widgets/itemviews/qtableview.cpp

@@ -595,7 +595,7 @@ void QTableViewPrivate::init()
     cornerWidget->setFocusPolicy(Qt::NoFocus);
     cornerWidgetConnection = QObject::connect(
           cornerWidget, &QTableCornerButton::clicked,
-          q, &QTableView::reset);
+          q, &QTableView::selectAll);
 #endif
 }