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