MB-10496: Add jemalloc as a cbdeps dependancy for macosx

Add jemalloc for OS X for use in memory defragmentation (as this is
the first platform which the cbdep has been created for).

Change-Id: I030a0edf24280f3db4edc8c7d6d2205819682818
Reviewed-on: http://review.couchbase.org/42900
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>