Belatedly track private lib renaming for OptionalObsoleteFiles.inc

Reviewed by: kevans
Fixes: 5551c573554e ("Rework PRIVATELIB")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32384

(cherry picked from commit 032448cd2c52161aa03fd4ee5bf243d78d61b53e)