make ns_server point to XMem prototype

Change-Id: I4e36201bd3e1967e953404e87828f2dbd4d1c95c
Reviewed-on: http://review.couchbase.org/26528
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>