Link with libxnet on HP-UX

Change-Id: I4c9279613e30d852fa9a82f6b41395eca6d9d027
Reviewed-on: http://review.couchbase.org/24539
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Yury Alioshinov <haster2010@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>