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.

Change-Id: Id85e3a72f53b85e6b155b224268e2fe587fe3406
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>