C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/mapgen.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_HEADER
-#define MAPGEN_HEADER
+#pragma once
 
 #include "noise.h"
 #include "nodedef.h"
@@ -299,5 +298,3 @@ protected:
 	float cavern_threshold;
 	int lava_depth;
 };
-
-#endif