kcoreaddons_desktop_to_json: note the generated file in the build log

Summary:
Being a known and explicitly generated build artifact, it helps
understanding the build process to see when the JSON file is generated.
It also creates consistency with similar build artifacts (e.g. source files
generated from UI files or JSON files created from kpackage desktop files).

Reviewers: apol

Reviewed By: apol

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10664