blkcg: Update MAINTAINERS entry

Josef wrote iolatency and iocost is missing from the files list. Let's add
Josef as a maintainer and add blk-iocost.c to the files list.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Jens Axboe <axboe@kernel.dk>

MAINTAINERS

@@ -5256,6 +5256,7 @@ F:	tools/testing/selftests/cgroup/
 
 CONTROL GROUP - BLOCK IO CONTROLLER (BLKIO)
 M:	Tejun Heo <tj@kernel.org>
+M:	Josef Bacik <josef@toxicpanda.com>
 M:	Jens Axboe <axboe@kernel.dk>
 L:	cgroups@vger.kernel.org
 L:	linux-block@vger.kernel.org
@@ -5263,6 +5264,7 @@ T:	git git://git.kernel.dk/linux-block
 F:	Documentation/admin-guide/cgroup-v1/blkio-controller.rst
 F:	block/bfq-cgroup.c
 F:	block/blk-cgroup.c
+F:	block/blk-iocost.c
 F:	block/blk-iolatency.c
 F:	block/blk-throttle.c
 F:	include/linux/blk-cgroup.h