Explicitly import Qt 5.7's QtQuick for Connections.enabled

Summary:
It was introduced in that version and apparently Qt 5.12 is stricter
about this and causes 'qrc:/package/contents/ui/main.qml:59
"Connections.enabled" is not available in QtQuick 2.1.' when trying
to start okularkirigami without this patch.

Test Plan: okularkirigami shows up

Reviewers: #okular, apol

Subscribers: okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D17196