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.

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>
(cherry picked from commit 6f1da57603cf86109e096001018ba9b0f1dd18b5)

Change-Id: Ib2d64abfe4b10af03d0d3d7b6550afa931ccd3a0
Reviewed-on: http://gerrit.openafs.org/10337
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>