ovs-numa.h: Add a missing OVS_UNUSED

ovs-numa.h: Add a missing OVS_UNUSED

Suppress the following warning:

> cc1: warnings being treated as errors
> In file included from ../lib/dpif.h:394:0,
>                  from ../lib/netdev.c:28:
> ../lib/ovs-numa.h: In function 'ovs_numa_dump_cores_on_numa':
> ../lib/ovs-numa.h:150:33: error: unused parameter 'numa_id'

The problem was introduced by
commit 9da2564e2bfa4ffc5a05552630ce2aca00a521c9.
("ovs-numa: Refine the module.")

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Ben Pfaff <blp@nicira.com>