D7154: Only require Qt5X11Extras when we actually need it

[Aleix Pol Gonzalez]

apol accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R274 KIdleTime

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D7154

To: vkrause, #frameworks, apol