tvolser: fix makefile clean target

Remove generated source files with the clean makefile target.

Reviewed-on: http://gerrit.openafs.org/10724
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit e39e226a13e38bfe0fb12b73633b6415c790c569)

Change-Id: Ie1159d2bfb05e274623b0985660e05ed9ce78ccc
Reviewed-on: http://gerrit.openafs.org/11231
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>