mission: add RTL to is_mission_item_reached_or_completed()

This ensures that a mission is marked as completed when RTL is the last item or that the mission can continue if there is an intermediate RTL.

src/modules/navigator/mission_block.cpp

@@ -96,6 +96,7 @@ MissionBlock::is_mission_item_reached_or_completed()
 	case NAV_CMD_SET_CAMERA_FOCUS:
 	case NAV_CMD_DO_CHANGE_SPEED:
 	case NAV_CMD_DO_SET_HOME:
+	case NAV_CMD_RETURN_TO_LAUNCH:
 
 		return true;