MAINTAINERS: Add Joel Granados as co-maintainer for proc sysctl

Joel Granados has been doing quite a bit of the work to help us move
forward with the proc sysctl cleanups, and is keen on helping and
so has agreed to help with maintenance of proc sysctl. Add him as
a maintainer.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>