Navigator: Fix return value typo for abs altitude

src/modules/navigator/mission.cpp

@@ -303,7 +303,7 @@ Mission::advance_mission()
 	}
 }
 
-int
+float
 Mission::get_absolute_altitude_for_item(struct mission_item_s &mission_item)
 {
 	if (_mission_item.altitude_is_relative) {

src/modules/navigator/mission.h

@@ -133,7 +133,7 @@ private:
 	 */
 	void altitude_sp_foh_reset();
 
-	int get_absolute_altitude_for_item(struct mission_item_s &mission_item);
+	float 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