netlink: specs: add phys-binding attr to rt_link spec

Add the missing phys-binding attr to the mctp-attrs in the rt_link spec.
This fixes commit 580db513 ("net: mctp: Expose transport binding
identifier via IFLA attribute").

Note that enum mctp_phys_binding is not currently uapi, but perhaps it
should be?

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20241213112551.33557-1-donald.hunter@gmail.com


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