Navigator: Make logic using previous and current altitudes consistent

src/modules/navigator/mission.h

@@ -132,6 +132,8 @@ private:
 	 */
 	void altitude_sp_foh_reset();
 
+	int get_absolute_altitude_for_item(struct mission_item_s &mission_item);
+
 	/**
 	 * Read current or next mission item from the dataman and watch out for DO_JUMPS
 	 * @return true if successful