MAINTAINERS: add block/rbd.c reviewer

adding myself as a designated reviewer.

Signed-off-by: Peter Lieven <pl@kamp.de>
Message-Id: <20210707180449.32665-2-pl@kamp.de>
Acked-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>