Correctly link to MIT KRB5 libraries when selected.

This was discovered while working through issues relating to an
exp-run using base with private Heimdal, part of the project to
make a) Heimdal in base private and b) import MIT into base (PR 222745).

PR: 227680
Submitted by: cy@
Approved by: Corey Halpin <chalpin@cs.wisc.edu> (maintainer)
MFH: 2018Q2