The commands qt6_add_executable and qt6_add_library both accept a list
of sources that get automatically added to the created target.
Extend qt6_add_plugin to also accept sources for consistency.
Fixes: QTBUG-104189
Change-Id: Iad5d8c5b31551663be155d068d55e770e1a91b27
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>