C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/itemdef.h

@@ -18,8 +18,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
-#ifndef ITEMDEF_HEADER
-#define ITEMDEF_HEADER
+#pragma once
 
 #include "irrlichttypes_extrabloated.h"
 #include <string>
@@ -174,5 +173,3 @@ public:
 };
 
 IWritableItemDefManager* createItemDefManager();
-
-#endif