efi: include endian.h to handle endian correctly

The macro __BYTE_ORDER__ is defined in endian.h.

src/boot/efi/sha256.h

@@ -1,6 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <endian.h>
+
 #include <efi.h>
 #include <efilib.h>