MAINTAINERS: add a general entry for BSD sockets

Create a MAINTAINERS entry for BSD sockets. List the top 3
reviewers as maintainers. The entry is meant to cover core
socket code (of which there isn't much) but also reviews
of any new socket families.

Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250202014728.1005003-3-kuba@kernel.org


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