Rather than listing block/monitor from the top-level Makefile.objs, we
should instead list monitor from block/Makefile.objs.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Fixes: bb4e58c6
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200608173339.3244211-1-eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>