Fix for memory leak in MAVLinkProtocol.

Found by valgrind.

src/comm/MAVLinkProtocol.cc

@@ -184,6 +184,7 @@ void MAVLinkProtocol::run()
     while(!_should_exit) {
 
         if (isFinished()) {
+            delete heartbeatTimer;
             qDebug() << "MAVLINK WORKER DONE!";
             return;
         }