C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/script/lua_api/l_util.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 L_UTIL_H_
-#define L_UTIL_H_
+#pragma once
 
 #include "lua_api/l_base.h"
 
@@ -100,5 +99,3 @@ public:
 
 	static void InitializeAsync(AsyncEngine &engine);
 };
-
-#endif /* L_UTIL_H_ */