Common: Move headers over to using pragma once

Just makes the headers more consistent.

Common/Common.h

@@ -15,8 +15,7 @@
 // Official SVN repository and contact information can be found at
 // http://code.google.com/p/dolphin-emu/
 
-#ifndef _COMMON_H_
-#define _COMMON_H_
+#pragma once
 
 // DO NOT EVER INCLUDE <windows.h> directly _or indirectly_ from this file
 // since it slows down the build a lot.
@@ -138,5 +137,3 @@ private:
 #endif
 
 #include "Swap.h"
-
-#endif // _COMMON_H_