Replace all include guard ifdefs with "#pragma once"

Source/Core/VideoCommon/Fifo.h

@@ -2,8 +2,7 @@
 // Licensed under GPLv2
 // Refer to the license.txt file included.
 
-#ifndef _FIFO_H
-#define _FIFO_H
+#pragma once
 
 #include "Common.h"
 #include "VideoBackendBase.h"
@@ -34,5 +33,3 @@ void Fifo_SetRendering(bool bEnabled);
 
 // Implemented by the Video Backend
 void VideoFifo_CheckAsyncRequest();
-
-#endif // _FIFO_H