AP_Terrain: replace header guard with pragma once

libraries/AP_Terrain/AP_Terrain.h

@@ -13,9 +13,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
-
-#ifndef __AP_TERRAIN_H__
-#define __AP_TERRAIN_H__
+#pragma once
 
 #include <AP_Common/AP_Common.h>
 #include <AP_HAL/AP_HAL.h>
@@ -410,4 +408,3 @@ private:
     enum TerrainStatus system_status = TerrainStatusDisabled;
 };
 #endif // AP_TERRAIN_AVAILABLE
-#endif // __AP_TERRAIN_H__