crypto: fix nettle config check for running pbkdf test

The pbkdf test is being built based on a check for CONFIG_NETTLE.
As of fff2f982ab6ac0dd2b641d30303f72270a019f28, it should be
instead checking CONFIG_NETTLE_KDF

Reported-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Tested-by: Bruce Rogers <brogers@suse.com>
Tested-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
