Fix distcheck make target

    CC       contrib/cbsasl/src/libcbsasl_la-client.lo
  ../contrib/cbsasl/src/client.c:20:31: fatal error: cram-md5/cram-md5.h: No such file or directory
  #include "cram-md5/cram-md5.h"
                                   ^

Change-Id: I065a32560b95fbf7f365eed2f89ad13f01b77274
Reviewed-on: http://review.couchbase.org/29739
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>