.mailmap: Add an entry for Marvin Haeuser

.mailmap: Add an entry for Marvin Haeuser

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

Add an entry for Marvin Häuser to have his/her name and email
address displayed properly in the git history (in particular,
commit 62ec4a5e).

Cc: Marvin Häuser <Marvin.Haeuser@outlook.de>
Cc: Marvin Häuser <Marvin.Haeuser@outlook.com>
Reviewed-by: Marvin Häuser <mhaeuser@outlook.de>
Signed-off-by: Philippe Mathieu-Daude <philmd@redhat.com>
Message-Id: <20200106115415.11624-20-philmd@redhat.com>
[lersek@redhat.com: translit subject to ASCII to suppress PatchCheck.py]