reciprocal_div: objects with exported symbols should be obj-y rather than lib-y

Otherwise the exported symbols might be discarded because of no users
in vmlinux.

Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>