MAINTAINERS: add Stefano Garzarella as io_uring reviewer

I've been working with io_uring for a while so I'd like to help
with reviews.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20210728131515.131045-1-sgarzare@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>