MAINTAINERS: add Simon as an official reviewer

Simon has been diligently and consistently reviewing networking
changes for at least as long as our development statistics
go back. Often if not usually topping the list of reviewers.
Make his role official.

Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Message-ID: <20241015153005.2854018-1-kuba@kernel.org>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>