C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/mapgen_carpathian.h

@@ -19,8 +19,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
-#ifndef MAPGEN_CARPATHIAN_HEADER
-#define MAPGEN_CARPATHIAN_HEADER
+#pragma once
 
 #include "mapgen.h"
 
@@ -101,5 +100,3 @@ private:
 	float terrainLevelAtPoint(s16 x, s16 z);
 	int generateTerrain();
 };
-
-#endif