Update mailmap

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

.mailmap

@@ -83,6 +83,7 @@ Mario Hros <spam@k3a.me>
 Mario Hros <spam@k3a.me> <root@k3a.me>
 Mario Macias <mariomac@gmail.com> <mmacias@newrelic.com>
 Mark Gordon <msg555@gmail.com>
+Marvin Giessing <marvin.giessing@gmail.com>
 Michael Crosby <crosbymichael@gmail.com> <michael@thepasture.io>
 Michael Katsoulis <michaelkatsoulis88@gmail.com>
 Mike Brown <brownwm@us.ibm.com> <mikebrow@users.noreply.github.com>