C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/content_nodemeta.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 CONTENT_NODEMETA_HEADER
-#define CONTENT_NODEMETA_HEADER
+#pragma once
 
 #include <iostream>
 
@@ -33,6 +32,3 @@ class IItemDefManager;
 void content_nodemeta_deserialize_legacy(std::istream &is,
 		NodeMetadataList *meta, NodeTimerList *timers,
 		IItemDefManager *item_def_mgr);
-
-#endif
-