Make sure to search for Qt5-based qmlplugindump

Summary: Without any hint, qmlplugindump version is whatever default is set by qtchooser.

Test Plan: ecm_find_qmlmodule now works properly for e.g. kirigami.

Reviewers: apol, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: aacid, dfaure, cgiboudeaux, Frameworks, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D9116