C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/guiKeyChangeMenu.h

@@ -19,8 +19,7 @@
  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#ifndef GUIKEYCHANGEMENU_HEADER
-#define GUIKEYCHANGEMENU_HEADER
+#pragma once
 
 #include "irrlichttypes_extrabloated.h"
 #include "modalMenu.h"
@@ -73,5 +72,3 @@ private:
 	gui::IGUIStaticText *key_used_text = nullptr;
 	std::vector<key_setting *> key_settings;
 };
-
-#endif