build: make ring mempool driver mandatory

build: make ring mempool driver mandatory

The default mempool driver is one based on the rte_ring, and as such it
needs to be present to have just about any app (which doesn't override
the mempool) run. Given this state of affairs it is probably best to add
this default mempool driver to the always-enabled list to ensure we get
a runnable build in all cases. This means that, for example, to run some
NIC tests with testpmd in a minimal build, in most cases, all the user
should need to do is specify "-Denable_drivers=net/<nic_drv>" for the
build.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>