Add missing XdgShellPopup::ackConfigure
When XdgShellV6 was retrofitted to match the XdgShellV5 API this got
missed in the client.
It went unnoticed as Kwin didn't handle configure events at the time.
Test Plan: Used in kwin unit tests
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: zzag, kde-frameworks-devel
Differential Revision: https://phabricator.kde.org/D18954