Fix KCompletionBox on wayland

Summary: On wayland it would immediately disappear, now it doesn't anymore.

Test Plan: Tried kcompletionuitest on xcb and wayland

Reviewers: Frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: anthonyfieroni

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D12448