security/pam-af: "Fix" missing pam_sm_authenticate().

Hacky workaround for

Jul  5 16:38:36 murthe sshd: in openpam_dispatch(): /usr/pkg/lib/security/pam_af.so: no pam_sm_authenticate()
Jul  5 16:38:36 murthe sshd: in openpam_check_error_code(): pam_sm_authenticate(): unexpected return value 2

See http://mail-index.netbsd.org/tech-pkg/2021/07/05/msg025227.html and
http://gnats.netbsd.org/39313.

(rhialto)