MAINTAINERS: Add include/block/aio.h to block I/O path section

This file is actually the header for async.c and aio-*.c., so add it to
the same section.

Suggested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1468826387-10473-1-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>