mavlink: publish GPS antenna heading offset as NAN

src/modules/mavlink/mavlink_receiver.cpp

@@ -2189,6 +2189,7 @@ MavlinkReceiver::handle_message_hil_gps(mavlink_message_t *msg)
 	hil_gps.satellites_used = gps.satellites_visible;  //TODO: rename mavlink_hil_gps_t sats visible to used?
 
 	hil_gps.heading = NAN;
+	hil_gps.heading_offset = NAN;
 
 	if (_gps_pub == nullptr) {
 		_gps_pub = orb_advertise(ORB_ID(vehicle_gps_position), &hil_gps);