AntennaTracker: fixed build with GPS changes

Tools/AntennaTracker/system.pde

@@ -52,7 +52,7 @@ static void init_tracker()
 	g_gps = &g_gps_driver;
 
     // GPS Initialization
-    g_gps->init(hal.uartB, GPS::GPS_ENGINE_STATIONARY);
+    g_gps->init(hal.uartB, GPS::GPS_ENGINE_STATIONARY, NULL);
 
     mavlink_system.compid = 4;
     mavlink_system.type = MAV_TYPE_ANTENNA_TRACKER;