KCoreAddons: fix compilation with qtbase git dev branch

Summary:

• Remove unneeded endl in qDebug-like statements
• Include <QList>, forward-decl doesn't work anymore, now that it's a typedef.
• Ifdef out local QStringList::operator+=(QString), QVector provides this in Qt 6

This happened more or less by accident, I don't plan on doing this
across more frameworks at this point.

Test Plan: Builds

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26009