heimdal: don't try to expose nonexistant symbols

For one reason or another these symbols aren't present so don't try to
make them available for linkage.

In the case of libroken these seem to be compatability bits we don't
need a thus don't compile. For others it seems to rot upstream, but
I've not investigated deeply.

Reviewed by: cy
Differential Revision: https://reviews.freebsd.org/D44246