integrity: Select CRYPTO from INTEGRITY_ASYMMETRIC_KEYS

Select CRYPTO from INTEGRITY_ASYMMETRIC_KEYS, since
INTEGRITY_ASYMMETRIC_KEYS selects several options that depend on CRYPTO.

This unblocks the removal of the CRYPTO selection from SIGNATURE.
SIGNATURE (lib/digsig.c) itself will no longer need CRYPTO, but
INTEGRITY_ASYMMETRIC_KEYS was depending on it indirectly via the chain
SIGNATURE => INTEGRITY_SIGNATURE => INTEGRITY_ASYMMETRIC_KEYS.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>