x11-themes/gtk-engines-adwaita is relying on implicit inherits of other eclasses to have prune_libtool_files function available. With the recent changes splitting out some functions, a cleanup may occur where this breaks such an implicit dependency. Such as versionator.eclass only needing estack which is how I found this particular package issue. Please add ltprune to inherit to prevent future QA errors.
Fixed in 3.22.3, to be stabled soon commit c953f4a57fadbc94c8aa59f2a75b6968b129d418 Author: Mart Raudsepp <leio@gentoo.org> Date: Thu Mar 23 14:07:37 2017 +0200 x11-themes/gtk-engines-adwaita: explicitly inherit ltprune for prune_libtool_files Gentoo-bug: 613342