crypto: sha256 - Add missing MODULE_LICENSE() to lib/crypto/sha256.c

lib/crypto/sha256.c / lib/crypto/libsha256.o may end up being a module,
so it needs a MODULE_LICENSE() line, add this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>