Common: Move header guards over to pragma once

Also replaced C headers with the C++ equivalent ones

src/common/atomic_gcc.h

@@ -2,8 +2,7 @@
 // Licensed under GPLv2
 // Refer to the license.txt file included.
 
-#ifndef _ATOMIC_GCC_H_
-#define _ATOMIC_GCC_H_
+#pragma once
 
 #include "common/common.h"
 
@@ -109,5 +108,3 @@ LONG SyncInterlockedExchange(LONG *Dest, LONG Val)
 #endif
 }
 #endif
-
-#endif