Fix parallel builds
Fix parallel builds

With the newly introduced generated sources in app/,
the (parallel) GNOME ci builds were failing. Add the
generated sources to BUILT_SOURCES to fix that. For
good measure, also add them to CLEAN_FILES.

Closes: #1843
Approved by: mwleeds

flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak