mission: keep 'offboard mission updated' printf as a warning

Useful to see when a in-flight mission update happens from the log.

src/modules/navigator/mission.cpp

@@ -329,6 +329,9 @@ Mission::update_offboard_mission()
 	bool failed = true;
 
 	if (orb_copy(ORB_ID(offboard_mission), _navigator->get_offboard_mission_sub(), &_offboard_mission) == OK) {
+		// The following is not really a warning, but it can be useful to have this message in the log file
+		PX4_WARN("offboard mission updated: dataman_id=%d, count=%d, current_seq=%d", _offboard_mission.dataman_id,
+			 _offboard_mission.count, _offboard_mission.current_seq);
 
 		/* determine current index */
 		if (_offboard_mission.current_seq >= 0 && _offboard_mission.current_seq < (int)_offboard_mission.count) {