rb_tree: silence unused function warnings

With DIAGNOSTIC set a kernel build generates warnings about the
defined-but-unused RB_RANK method. Don't set _RB_DIAGNOSTIC
automatically, to silence these warnings.

Reported by: mjguzik@gmail.com
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D36617

(cherry picked from commit 14696d81f163dacfcdaf42171556ebfd15416fb9)