[chmodjob] Port away from QLinkedList

Summary: QLinkedList is likely to go away in Qt 6. Use a std::stack instead

Test Plan: Changed file permissions in Dolphin, verified with ls -l

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: dfaure, bruns, fabiank, kde-frameworks-devel

Tags: Frameworks

Maniphest Tasks: T12087

Differential Revision: https://phabricator.kde.org/D25443