Set the script source for KReportDesigner and KReportPreRenderer in the example

Summary:
KReportExampleDataSource has a sample script that was not available from the
designer because it was not set as its script source; the same goes for the
pre-renderer.

Reviewers: staniek

Reviewed By: staniek

Subscribers: staniek

Tags: KReport

Differential Revision: https://phabricator.kde.org/D28763