Make sample shell include work with custom Qt

Summary: This makes the shell include work with the prebuilt Qt from qt.io, saving users with less powerful machines a lot of time.

Test Plan: applications can be compiled against the Qt binaries from qt.io and launch.

Reviewers: kdesrc-build, mpyne

Reviewed By: kdesrc-build, mpyne

Subscribers: mpyne

Differential Revision: https://phabricator.kde.org/D21387