Add LICENSE.txt to gutil

Change-Id: Ibce51ed520d3c99dce70ec74adfec45a09067ac4
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5164
Reviewed-by: Todd Lipcon <todd@cloudera.com>
Reviewed-by: Henry Robinson <henry@cloudera.com>
Tested-by: Henry Robinson <henry@cloudera.com>