vol: Fix build with separate objdir

The volscan-main and volinfo-main source files are in the source
tree, not the object tree; refer to the objects in the Makefile
as dependencies, so that they will be picked up properly.  The
objects will be made just fine by the implicit .c.o rule.

Reviewed-on: http://gerrit.openafs.org/10988
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 074d745a02d80bfd2c16a4e2b7b4222022f8e641)

Change-Id: I3f29fcfb0257e145e0aae62ba425ccd7df513a72
Reviewed-on: http://gerrit.openafs.org/11279
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>