D17015: Fix the Qt doc creation with Qt 5.12.

[Christophe Giboudeaux]

cgiboudeaux added a reviewer: kossebau.

REPOSITORY
  R240 Extra CMake Modules

REVISION DETAIL
  https://phabricator.kde.org/D17015

To: cgiboudeaux, kossebau
Cc: kde-frameworks-devel, kde-buildsystem, michaelh, ngraham, bruns