fix duplicated symbols compilation error with mingw on Windows

Summary: fix duplicated symbols compilation error with mingw on Windows

Test Plan: Tested with mingw-5.3 under windows 7 and on linux with gcc

Reviewers: kfunk, Windows

Reviewed By: kfunk, Windows

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5443