navigator: audio messages about what is happening, RTL during mission not triggered but after mission

src/modules/navigator/mission.h

@@ -151,6 +151,11 @@ private:
 	 */
 	void report_current_offboard_mission_item();
 
+	/**
+	 * Report that the mission is finished if one exists or that none exists
+	 */
+	void report_mission_finished();
+
 	/**
 	 * Publish the mission result so commander and mavlink know what is going on
 	 */