FramebufferManager: Add missing header guard

Source/Core/VideoBackends/Null/FramebufferManager.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2+
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include "VideoCommon/FramebufferManagerBase.h"
 
 class XFBSource : public XFBSourceBase