Re-add KWindowEffectsPrivateV2 destructor
32810daa removed this, which for some reason made LibreOffice crash on launch for me. Re-adding it resolves that issue.
Note that this is quite far outside of my area of expertise, so I have zero ability to justify or depend this change. :) I'm simply submitting it since @dfaure mentioned it as a potential fix in 32810daa (comment 299164), and it works for me.