manifest: Use latest meta-qt5 from github.com
The "official" open soucre variant of meta-qt5 is gitgub anyways. And as the latest 5.15 version from code.qt.io is not working for imx6ull and ima8m pull the latest version from github.
To do this add the github.com/meta-qt5 as remote
BCS 746-000364
Change-Id: Ie7daec31b56dfd4b578d8c8ec62cc0959b02e1ea