MAINTAINERS: Update sysctl tree location

To more efficiently co-maintain the sysctl subsystem a shared repository
has been created at https://git.kernel.org/pub/scm/linux/kernel/git/sysctl/sysctl.git
and the sysctl-next branch that Luis Chamberlain (mcgrof@kernel.org) maintained at
https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux.git/log/?h=sysctl-next
has moved to the shared sysctl-next branch located at
https://git.kernel.org/pub/scm/linux/kernel/git/sysctl/sysctl.git/log/?h=sysctl-next

.
This commit changes the sysctl tree in MAINTAINERS to reflect this change.

Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Joel Granados <j.granados@samsung.com>