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