Common: Add missing header guards

Source/Core/Common/GL/GLExtensions/GLExtensions.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2+
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include <string>
 
 #include "Common/CommonTypes.h"

Source/Core/Common/VariantUtil.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2+
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include <variant>
 
 namespace detail