D22990: Fix build with older cmake

[Sebastian Gottfried]

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

REPOSITORY
  R336 KTouch

REVISION DETAIL
  https://phabricator.kde.org/D22990

To: wbauer, gottfried
Cc: kde-edu, #kde_applications, fbampaloukas, alexeymin, genaxxx, jriddell, narvaez, apol
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