pathstroke example: Add Q_FALLTHROUGH to unmarked fallthrough seen by GCC 7

Task-number: QTBUG-60635
Change-Id: I10695f96122f1b0859bbe7fadb349efb3c7277a6
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

examples/widgets/painting/pathstroke/pathstroke.cpp

@@ -600,6 +600,7 @@ bool PathStrokeRenderer::event(QEvent *e)
     switch (e->type()) {
     case QEvent::TouchBegin:
         touchBegin = true;
+        Q_FALLTHROUGH();
     case QEvent::TouchUpdate:
     {
         const QTouchEvent *const event = static_cast<const QTouchEvent*>(e);