MAINTAINERS: Add Luis Chamberlain as modules maintainer

Luis has kindly agreed to help maintain the module loader. As my
responsibilities have shifted, I've found myself with less cycles to
devote to upstream maintenance these days. Luis is already very involved
and engaged upstream, and with his experience maintaining the kmod
module loader and usermode helper, I believe he is a great fit for this
area of the kernel.

Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Jessica Yu <jeyu@kernel.org>