bootparamd: Add missing __unused mark.

e03764d931d820185a019334259b18df2e3f6b6c did not catch all unused
variables.

Submitted by: otis
Differential Revision: https://reviews.freebsd.org/D27894

(cherry picked from commit 8c45fe5d8ecda4be7564aadaa50712790c6c0a6f)