kerberos5: Avoid embedding full paths in generated files

[markj]

Summary

When reproducible builds are enabled, we want to use /usr/src and
/usr/obj as the canonical src and obj paths.

Various files generated by the Heimdal Kerberos builds embed the
original source path in generated files. It doesn't look like the
utilities which generate those files can be taught to perform
substitution, so the easiest thing to do is fix them up after the fact.