Fix building autotest on platforms where syscalls are not available

Summary:
On arm64 e.g SYS_open and few more syscalls are not available, this
results in build failure.

Test Plan: builds locally

Reviewers: #plasma, fvogt, graesslin, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidk, graesslin, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9225