Always include lcbuv source code in dist

Change-Id: I449fa1a0548ebf67992db7feb695823f791cc7ab
Reviewed-on: http://review.couchbase.org/28286
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>