xcb: make sure to update window title when it is changed

Task-number: QTBUG-33775
Change-Id: Ibe346c4304532d031bfa1c9b4d73cd02d1e5a64f
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -1342,6 +1342,7 @@ void QXcbWindow::setWindowTitle(const QString &title)
                                    8,
                                    ba.length(),
                                    ba.constData()));
+    xcb_flush(xcb_connection());
 }
 
 void QXcbWindow::setWindowIcon(const QIcon &icon)