nvme: add Guenther as nvme-hwmon maintainer

Given that non of the overall NVMe maintainers knows this code very
deeply it probably makes sense to add Guenther as an additional
MAINTAINER for it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Acked-by: Guenter Roeck <linux@roeck-us.net>