Fix crash on exit in kio_file

Summary:
All QTextCodec are deleted by QCoreGlobalData on exit, so they must be allocated on the heap.
Before Qt 5.12, it is even not allowed to delete them.

BUG: 408797

Reviewers: cfeck, dfaure, broulik

Reviewed By: dfaure

Subscribers: aacid, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D24150