media: mailmap: add entry for Michael Riesch

After five interesting years, I left WolfVision and started to work for
Collabora. Add a corresponding mailmap entry.

Signed-off-by: Michael Riesch <michael.riesch@collabora.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>