Provide alternative Heimdal pkg-config example

Provide alternative Heimdal pkg-config example

Building Exim against Heimdal 1.5, heimdal-gssapi.pc was needed.
There's been a major version bump in Heimdal, and against 7.1 that
doesn't work; using heimdal-krb5.pc fails on missing `gss_*` functions.

I can find no build documentation for Heimdal which describes what
should be needed.  heimdal-gssapi.pc does reference heimdal-krb5.pc
in `Requires.private` but it's not being used by FreeBSD pkgconf in
such a way that it's available when building on FreeBSD 10.3.

Fortunately, our `*_PC` logic works with multiple packages listed,
so provide that example.