C++ modernize: Pragma once (#6264)

* Migrate cpp headers to pragma once

src/irr_aabb3d.h

@@ -17,13 +17,10 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */
 
-#ifndef IRR_AABB3D_HEADER
-#define IRR_AABB3D_HEADER
+#pragma once
 
 #include "irrlichttypes.h"
 
 #include <aabbox3d.h>
 
 typedef core::aabbox3d<f32> aabb3f;
-
-#endif