security/sssd: Fix package with SMB=on

While here, add comment in security/krb5 to remember the obscure dependency in
security/sssd so it does not break again.

PR: 244778
Reported by: tommyhp2@gmail.com
Tested by: tommyhp2@gmail.com
MFH: 2021Q2 (build fix)

(cherry picked from commit 11964e74b9165c24b41ac8e6960f727d36ac4241)