xwl: Fix unused warning in xwldrophandler.cpp

src/xwl/xwldrophandler.cpp

@@ -42,6 +42,7 @@ bool XwlDropHandler::handleClientMessage(xcb_client_message_event_t *event)
 
 void XwlDropHandler::updateDragTarget(KWaylandServer::SurfaceInterface *surface, quint32 serial)
 {
+    Q_UNUSED(serial)
     auto client = workspace()->findClient([surface](const X11Client *c) {return c->surface() == surface;});
     if (m_xvisit && client == m_xvisit->target()) {
         return;