C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/map.h

@@ -17,8 +17,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
-#ifndef MAP_HEADER
-#define MAP_HEADER
+#pragma once
 
 #include <iostream>
 #include <sstream>
@@ -525,5 +524,3 @@ protected:
 	*/
 	std::map<v3s16, u8> m_loaded_blocks;
 };
-
-#endif