[rel-2.2.0] freeze libvbucket at pre-cmake change

So that build is not broken

Change-Id: Ib248f42d0d1509f9f3389146da616b171e7aa4ce
Reviewed-on: http://review.couchbase.org/27977
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>