D9673: Guard against syscalls not available on some platforms

[David Kahles]

davidk added reviewers: bshah, graesslin.

REPOSITORY
  R133 KScreenLocker

REVISION DETAIL
  https://phabricator.kde.org/D9673

To: davidk, bshah, graesslin
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart