C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/settings.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 SETTINGS_HEADER
-#define SETTINGS_HEADER
+#pragma once
 
 #include "irrlichttypes_bloated.h"
 #include "util/string.h"
@@ -234,6 +233,3 @@ private:
 	mutable std::mutex m_mutex;
 
 };
-
-#endif
-