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>

MAINTAINERS

@@ -2695,6 +2695,7 @@ F:	drivers/connector/
 CONTROL GROUP (CGROUP)
 M:	Tejun Heo <tj@kernel.org>
 M:	Li Zefan <lizefan@huawei.com>
+M:	Johannes Weiner <hannes@cmpxchg.org>
 L:	cgroups@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git
 S:	Maintained