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.

Change-Id: Ieec4b32cfbe5d260e1560a08d4ed8162720f9222
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>