MAINTAINERS: add a cgroup core co-maintainer

Add Johannes Weiner as a co-maintainer of cgroup core.  Johannes has
been heavily involved with the design of the v2 interface and is
familiar with design and implementation of cgroup core.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>