Fix create_symlink path to DESTDIR instead of root file system
Test Plan: Builds fine again in a sandbox, with the symlink properly installed.
Reviewers: Dolphin, elvisangelaccio
Reviewed By: Dolphin, elvisangelaccio
Subscribers: davidedmundson, elvisangelaccio, kfm-devel, ngraham
Tags: Dolphin
Differential Revision: https://phabricator.kde.org/D21011