D18941: Fix build with cmake 3.5

[Luca Beltrame]

lbeltrame added reviewers: Build System, cgiboudeaux.

REPOSITORY
  R286 KFileMetaData

REVISION DETAIL
  https://phabricator.kde.org/D18941

To: wbauer, #build_system, cgiboudeaux
Cc: kde-frameworks-devel, #baloo, ashaposhnikov, michaelh, astippich, spoorun, ngraham, bruns, abrahams
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