allow for unchanged objects

There are situations in which an object may be rebuilt but have no
functional changes, such as a change to an included header file.

This commit changes kpatch-build to tolerate individual unchanged
objects so long as there is, in the end, at least one changed object.

Signed-off-by: Seth Jennings <sjenning@redhat.com>
Merge pull request #348 from spartacus06/allow-unchanged-object

allow for unchanged objects