ndp: Fix libxo formatting for the header of neighbor cache

PR: 272749
Reviewed by: zlei
Fixes: e1c7783e220b ndp(8): add structured output formatting via libxo
MFC after: 3 days

(Note this is only a partial MFS, as the netlink part was not MFS'ed)

(cherry picked from commit 209905ec384eedd58f1b9b7ab774029d22787dfb)
(cherry picked from commit 07cf1ce012c51e30ef5078c23a7d68a9b5d87c0a)