Tablet example: Fix warning about switch/fallthrough

Add Q_FALLTHROUGH(), fixing:
tabletcanvas.cpp:222:55: warning: this statement may fall through [-Wimplicit-fallthrough=]

Change-Id: I573f4add56774218e28a7c5f1dac09c5402333c0
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

examples/widgets/widgets/tablet/tabletcanvas.cpp

@@ -224,7 +224,7 @@ void TabletCanvas::paintPixmap(QPainter &painter, QTabletEvent *event)
                 qWarning() << error;
 #endif
             }
-            // FALL-THROUGH
+            Q_FALLTHROUGH();
         case QTabletEvent::Stylus:
             painter.setPen(m_pen);
             painter.drawLine(lastPoint.pos, event->posF());