dm integrity: select CRYPTO_SKCIPHER

The integrity target relies on skcipher for encryption/decryption, but
certain kernel configurations may not enable CRYPTO_SKCIPHER, leading to
compilation errors due to unresolved symbols. Explicitly select
CRYPTO_SKCIPHER for DM_INTEGRITY, since it is unconditionally dependent
on it.

Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>