lindebugfs: use func not FUNCTION

Change to the C99 version and not the old gcc name.
No functional changes.

Sponsored by: The FreeBSD Foundation
Reviewed by: dumbbell, emaste
Differential Revision: https://reviews.freebsd.org/D48736

(cherry picked from commit 94e6c8d7d19de68b29233e59c85b16c645c1a872)