mk: link app with ring mempool by default

mk: link app with ring mempool by default

From the discussion in [1], it was observed that application should
have a default pool already linked even in case of shared builds.
Ring is especially important because packet mbuf creation API refer to
ring_mp_mc as default handler.

Documentation for this is pending.

[1] http://dpdk.org/ml/archives/dev/2017-April/063819.html

Fixes: 9a8e9b57f544 ("mempool: move ring handler as a driver")

Signed-off-by: Shreyansh Jain <shreyansh.jain@nxp.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>