security/sssd: Fix pkg-plist to include PAC files

In PR 244778 this port was reported to fail during package. sssd_pac and others
were not generated by the build process. They were removed from the pkg-plist
and the issue closed (maintainer timed out).

Recently joerg@ reported sssd_pac should be included. It turns out,
files/patch-src_external_pac__responder.m4 needs to be updated whenever a
version bump of security/krb5 occurs[1]. This is kind of obscure since building
security/sssd with default options does not reproduce the problem (SMB=on is
needed).

[1] https://svnweb.freebsd.org/changeset/ports/526479

PR: 244778
Reported by: joerg@
Approved by: maintainer (timeout)
MFH: 2020Q3 (plist fix)