lib: digital signature dependency fix

Randy Dunlap reported build break:

ERROR: "crypto_alloc_shash" [lib/digsig.ko] undefined!
ERROR: "crypto_shash_final" [lib/digsig.ko] undefined!
ERROR: "crypto_shash_update" [lib/digsig.ko] undefined!
ERROR: "crypto_destroy_tfm" [lib/digsig.ko] undefined!

Added CRYPTO dependency and selected SHA1 algorithm.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: James Morris <jmorris@namei.org>