MAINTAINERS: 9p: add Christian Schoenebeck as reviewer

Volunteering as reviewer for 9p patches. As I am quite familiar with the
9p code base in the Linux kernel already, plus being current maintainer
of 9p in QEMU this move probably makes sense.

Link: https://lkml.kernel.org/r/E1n4jXv-000445-GK@lizzy.crudebyte.com


Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
[Dominique: reworded description]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>