bos: Change the remaining use of ktc_to_charptr to char cast

This is a 1.6 only change to complement the preceding commit.

On the master branch, this modification is part of commit
a9b8fd81bf14c04d33113e9e4df4e1ca7573e49d which otherwise comprises
changes not foreseen to be applied to 1.6.

Change-Id: I44678c2fb15f136d70f2d38a3996ad3d7d0455d5
Reviewed-on: http://gerrit.openafs.org/10891
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>