MAINTAINERS: add Kuniyuki Iwashima to TCP reviewers

List Kuniyuki as an official TCP reviewer.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250202014728.1005003-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>