Fix autotests on the FreeBSD CI.

Summary: Include <errno.h> explicitly, for EPROTO.

Reviewers: bcooksley, graesslin, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel, #frameworks

Tags: #plasma_on_wayland, #frameworks

Differential Revision: https://phabricator.kde.org/D5717