MB-6595 Install mlockall stub correctly.

On platforms where the actual NIF is not built.

Change-Id: Iec69c19fefc016d451013a6fb649146b7fdcc7ae
Reviewed-on: http://review.couchbase.org/23993
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>