netlink: specs: rt-link: add notification for newlink

Add a notification entry for netlink so that we can test ntf handling
in classic netlink and C.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250418021706.1967583-9-kuba@kernel.org


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