C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/server.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 SERVER_HEADER
-#define SERVER_HEADER
+#pragma once
 
 #include "network/connection.h"
 #include "irr_v3d.h"
@@ -654,5 +653,3 @@ private:
 	Shuts down when kill is set to true.
 */
 void dedicated_server_loop(Server &server, bool &kill);
-
-#endif