[util] Move enum_bitfields into util

Continuing on the cleaning up of the src/include 'junk drawer', the
enum_bitfields.h and its dependency basic_types.h are now in util.

src/kernel/gdt.h

@@ -1,9 +1,9 @@
 #pragma once
 /// \file gdt.h
 /// Definitions relating to a CPU's GDT table
-#include <stdint.h>
 
-#include "enum_bitfields.h"
+#include <stdint.h>
+#include <util/enum_bitfields.h>
 
 class TSS;