mavlink_mission: support INT altitude frames

The relative altitude flag was not set correclty when waypoints are sent
using the INT protocol.

src/modules/mavlink/mavlink_mission.cpp

@@ -927,10 +927,12 @@ MavlinkMissionManager::parse_mavlink_mission_item(const mavlink_mission_item_t *
 
 		mission_item->altitude = mavlink_mission_item->z;
 
-		if (mavlink_mission_item->frame == MAV_FRAME_GLOBAL) {
+		if (mavlink_mission_item->frame == MAV_FRAME_GLOBAL ||
+		    mavlink_mission_item->frame == MAV_FRAME_GLOBAL_INT) {
 			mission_item->altitude_is_relative = false;
 
-		} else if (mavlink_mission_item->frame == MAV_FRAME_GLOBAL_RELATIVE_ALT) {
+		} else if (mavlink_mission_item->frame == MAV_FRAME_GLOBAL_RELATIVE_ALT ||
+			   mavlink_mission_item->frame == MAV_FRAME_GLOBAL_RELATIVE_ALT_INT) {
 			mission_item->altitude_is_relative = true;
 		}