Replace all include guard ifdefs with "#pragma once"

Source/Core/Core/MemTools.h

@@ -2,8 +2,7 @@
 // Licensed under GPLv2
 // Refer to the license.txt file included.
 
-#ifndef _MEMTOOLS_H
-#define _MEMTOOLS_H
+#pragma once
 
 #include "CommonTypes.h"
 
@@ -13,5 +12,3 @@ namespace EMM
 	typedef u32 EAddr;
 	void InstallExceptionHandler();
 }
-
-#endif