Get rid of warning when building qplatformwindow_qpa.cpp.

This Q_UNUSED was unintentionally lost in 55fa3c189f.

Change-Id: I832ee7c8d46cf66f7b63045015bef181fddebeef
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>

src/gui/kernel/qplatformwindow_qpa.cpp

@@ -147,6 +147,7 @@ QMargins QPlatformWindow::frameMargins() const
 */
 void QPlatformWindow::setVisible(bool visible)
 {
+    Q_UNUSED(visible);
     QRect rect(QPoint(), geometry().size());
     QWindowSystemInterface::handleSynchronousExposeEvent(window(), rect);
 }