D5443: fix duplicated symbols compilation error with mingw on Windows

[Kevin Funk]

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

REPOSITORY
  R286 KFileMetaData

BRANCH
  windows_fix

REVISION DETAIL
  https://phabricator.kde.org/D5443

To: mgallien, kfunk, #windows
Cc: #frameworks