C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/metadata.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 METADATA_HEADER
-#define METADATA_HEADER
+#pragma once
 
 #include "irr_v3d.h"
 #include <iostream>
@@ -57,5 +56,3 @@ protected:
 	StringMap m_stringvars;
 
 };
-
-#endif