volser: add missing Makefile dependencies

volser: add missing Makefile dependencies

Add some missing dependencies in the volser Makefile to prevent
trying to compile while a header file is still being generated.

This caused parallel make errors on some slower systems.

Change-Id: Ieab26cf03a86e340f442d683512a7326eec7834d
Reviewed-on: http://gerrit.openafs.org/3244
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>