Enable dvolser and tvolser for FreeBSD

The top-level Makefile did not recurse into dvolser or tvolser for
FreeBSD.  They compile just fine, so enable them.

Reviewed-on: http://gerrit.openafs.org/9598
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit e93360ecc79822dd8149990a6eb5f3235b7cca1f)

Change-Id: Ia7dfbce23453a147f2c1ba81718373f632a039b6
Reviewed-on: http://gerrit.openafs.org/10122
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>