kpatch-build: prevent die if only part of objects have no change

Consider following patch: https://lkml.org/lkml/diff/2014/1/7/637/1
Kpatch-build will generate two objects for it. however mlock.o has no
changed function and will cause kpatch-build die.
Signed-off-by: Madper Xie <cxie@redhat.com>