Mark ManagerDataPaths as a proper export

Patch from Jean-Michaël Celerier
Thanks a lot

Summary: It is being used in main.cpp (ManagerDataPaths::self()) so needs exporting even when tests aren't enabled.

Reviewers: mlaurent, dfaure, vkrause

Reviewed By: mlaurent, dfaure

Subscribers: vkrause, dfaure, mlaurent

Differential Revision: https://phabricator.kde.org/D22575