.mailmap: Add an entry for Antoine Coeur

.mailmap: Add an entry for Antoine Coeur

We use .mailmap to display contributors email addresses in an
uniform format.

Add an entry for Antoine Cœur to have his/her name and email
address displayed properly in the git history.

Cc: Antoine Cœur <coeur@gmx.fr>
Reviewed-by: Antoine Cœur <coeur@gmx.fr>
Signed-off-by: Philippe Mathieu-Daude <philmd@redhat.com>
Message-Id: <20200106115415.11624-4-philmd@redhat.com>
[lersek@redhat.com: fix up valid PatchCheck.py error in Reviewed-by]
[lersek@redhat.com: translit subject to ASCII to suppress PatchCheck.py]