fix QMutex warning in tst_QLocalSocket::threadedConnection

Change-Id: I5728af1944e44bd976e51d4f0c0a923deacbeea0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>

tests/auto/network/socket/qlocalsocket/tst_qlocalsocket.cpp

@@ -717,6 +717,7 @@ void tst_QLocalSocket::threadedConnection()
     server.mutex.lock();
     server.start();
     server.wc.wait(&server.mutex);
+    server.mutex.unlock();
 
     QList<Client*> clients;
     for (int i = 0; i < threads; ++i) {