The lastest version of dash doesn't use warnx anymore, so no need to include err.h either. Since this headers isn't available on some platforms, the patch currently breaks the build there. The patch can be dropped. Thanks!
commit b469f19ca5eddefc9330b0913751b4d100dcdf8c Author: Lars Wendler <polynomial-c@gentoo.org> Date: Tue Aug 22 14:22:44 2017 app-shells/dash: Revbump to drop warnx patch (bug #628588). Package-Manager: Portage-2.3.8, Repoman-2.3.3