build: Include built files from app/
build: Include built files from app/

This fixes an out-of-tree build.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #1848
Approved by: alexlarsson

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