mailmap: add athanatos <sam.just@inktank.com>

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #1089 from dachary/wip-mailmap

mailmap: add athanatos <sam.just@inktank.com>

Reviewed-by: Samuel Just <sam.just@inktank.com>