MAINTAINERS: bfq: Add Paolo as maintainer for the BFQ I/O scheduler

Seems like this was forgotten in the bfq-series from Paolo. Let's do it now
so people don't miss out involving Paolo for any future changes or when
reporting bugs.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Jens Axboe <axboe@fb.com>