cleanup: get rid of the legacy #ifndef #define craziness

source/Irrlicht/os.h

@@ -2,8 +2,7 @@
 // This file is part of the "Irrlicht Engine".
 // For conditions of distribution and use, see copyright notice in irrlicht.h
 
-#ifndef __IRR_OS_H_INCLUDED__
-#define __IRR_OS_H_INCLUDED__
+#pragma once
 
 #include "irrTypes.h"
 #include "irrString.h"
@@ -95,7 +94,3 @@ namespace os
 
 } // end namespace os
 } // end namespace irr
-
-
-#endif
-