MB-100: modify junyi's toybuild manifest file for xmem

Change-Id: I94ef62814bdec334249ea1be26f1b0e69539c357
Reviewed-on: http://review.couchbase.org/26444
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>