Replace all include guard ifdefs with "#pragma once"

Source/Core/Core/ARDecrypt.h

@@ -2,8 +2,7 @@
 // Licensed under GPLv2
 // Refer to the license.txt file included.
 
-#ifndef _ARDECRYPT_H_
-#define _ARDECRYPT_H_
+#pragma once
 
 #include <vector>
 #include "Common.h"
@@ -15,5 +14,3 @@ namespace ActionReplay
 void DecryptARCode(std::vector<std::string> vCodes, std::vector<AREntry> &ops);
 
 } //namespace
-
-#endif //_ARDECRYPT_H_