D9408: extractors: Hide warnings from system headers

[Kevin Funk]

kfunk added a reviewer: mgallien.

REPOSITORY
  R286 KFileMetaData

REVISION DETAIL
  https://phabricator.kde.org/D9408

To: kfunk, mgallien
Cc: #frameworks, ashaposhnikov, michaelh, astippich, spoorun, bruns