relink binary whenever config-devices.mak changes:
this makes sense as we are adding/removing devices,
so binary has to be relinked to be up to date.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <1424332114-13440-2-git-send-email-mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>