D9330: Fix cmake warning (with cmake >= 3.9.0)

[Laurent Montel]

mlaurent added a comment.


  ok see https://phabricator.kde.org/D9334

REPOSITORY
  R241 KIO

REVISION DETAIL
  https://phabricator.kde.org/D9330

To: mlaurent, dfaure
Cc: kossebau, flherne, #frameworks