MAINTAINERS: add entry for UNIX sockets

Add a MAINTAINERS entry for UNIX socket, Kuniyuki has been
the de-facto maintainer of this code for a while.

Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250202014728.1005003-4-kuba@kernel.org


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