ndp: Fix libxo formatting for the prefix expiry string

Fixes: e1c7783e220b ("ndp(8): add structured output formatting via libxo")
MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Bell Tower Integration

(cherry picked from commit b3877767883e9b26fbcce47c015ba3eab323bf1b)