Tracker: fix sim_vehicle.py with --tracker option

AntennaTracker/system.cpp

@@ -27,6 +27,10 @@ void Tracker::init_ardupilot()
 
     // setup telem slots with serial ports
     gcs().setup_uarts();
+    // update_send so that if the first packet we receive happens to
+    // be an arm message we don't trigger an internal error when we
+    // try to initialise stream rates in the main loop.
+    gcs().update_send();
 
 #if LOGGING_ENABLED == ENABLED
     log_init();