boot: Use stddef.h offsetof

(cherry picked from commit 11f9a32)

Author: medhefgo

src/boot/efi/util.h

@@ -3,11 +3,10 @@
 
 #include <efi.h>
 #include <efilib.h>
+#include <stddef.h>
 
 #include "string-util-fundamental.h"
 
-#define offsetof(type, member) __builtin_offsetof(type, member)
-
 #define UINTN_MAX (~(UINTN)0)
 #define INTN_MAX ((INTN)(UINTN_MAX>>1))
 #ifndef UINT32_MAX