Akonadi searches for file mysql-global.conf at wrong path

Akonadi can't find mysql-global.conf placed at path /etc/xdg/akonadi dir.
It looks for it at /etc/xdg/akonadi/akonadi/ (see extra akonadi/ part appended).

BUG: 422079

Differential Revision: https://phabricator.kde.org/D29833