mailmap: add entries for Mathieu Othacehe

Add my gnu.org mail address.

Link: https://lkml.kernel.org/r/20231223144226.25740-1-othacehe@gnu.org
Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>
Cc: Bjorn Andersson <quic_bjorande@quicinc.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Konrad Dybcio <konrad.dybcio@linaro.org>
Cc: Matthieu Baerts <matttbe@kernel.org>
Cc: Matt Ranostay <matt@ranostay.sg>
Cc: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>