mavlink_mission: round lat/lon

This avoids casting errors. An alternative to `std::round` would be to
add 0.5 before casting for positive values and subtract 0.5 before
casting for negative values.

src/modules/mavlink/mavlink_mission.cpp

@@ -1560,8 +1560,8 @@ MavlinkMissionManager::format_mavlink_mission_item(const struct mission_item_s *
 			mavlink_mission_item_int_t *item_int =
 				reinterpret_cast<mavlink_mission_item_int_t *>(mavlink_mission_item);
 
-			item_int->x = (int32_t)(mission_item->lat * 1e7);
-			item_int->y = (int32_t)(mission_item->lon * 1e7);
+			item_int->x = std::round(mission_item->lat * 1e7);
+			item_int->y = std::round(mission_item->lon * 1e7);
 
 		} else {
 			mavlink_mission_item->x = (float)mission_item->lat;