Add ns_server/src to rebar.config source dirs.
Add ns_server/src to rebar.config source dirs.

Otherwise only tests get built by new rebar.

Change-Id: I78af2e078cb2a24587fd2886e37abb9e7fd59611
Reviewed-on: http://review.couchbase.org/71813
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server