sha256: header needs stddef

The sha256 header uses size_t which is within stddef, so add it.

Signed-off-by: William Roberts <william.c.roberts@intel.com>

src/fundamental/sha256.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include <stddef.h>
 #include <stdint.h>
 
 #define SHA256_DIGEST_SIZE 32