libgcrypt: Declare the sha256 shaext function

libgcrypt: Declare the sha256 shaext function

There is no prototype of _gcry_sha256_transform_intel_shaext() defined
in the header or libgcrypt-grub/cipher/sha256.c, and gcc may complain
the missing-prototypes error when compiling sha256-intel-shaext.c.

Declare the prototype in sha256-intel-shaext.c to avoid the error.

Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>