MAINTAINERS: add networking entries for Willem

We often have to ping Willem asking for reviews of patches
because he doesn't get included in the CC list. Add MAINTAINERS
entries for some of the areas he covers so that ./scripts/ will
know to add him.

Acked-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Link: https://lore.kernel.org/r/20230117190141.60795-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>