tvolser: fix makefile clean target

Remove generated source files with the clean makefile target.

Change-Id: I1e7d06c217f63fb9ee749e23bca1531d22babdda
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>