Log Message:
security/heimdal: provide krb5-gssapi.pc as symlink

This is needed for example for qt5-qtbase to pick up a pkgsrc-installed
heimdal instead of possibly a mix of system mit-krb5 libs with pkgsrc
headers, for its network auth that recently got GSSAPI.

It makes sense to provide the same pkg-config package name if heimdal and
mit-krb5 should be transparently compatible at that front.