Link against libz.dylib
Link against libz.dylib

because iErl no longer includes the zlib sources.

Change-Id: I01ae34112e075b8027a525495a547277c50ea0f8
Reviewed-on: http://review.couchbase.org/9609
Reviewed-by: Chris Anderson <2d93822289c316cb3d36491aa561290e86ec1ac5@couchbase.com>
Tested-by: Jens Alfke <21e2afd93d5ce53123476032b5dca8a79b546094@couchbase.com>

couchbase/iOS-Couchbase,couchbase/iOS-Couchbase