Appdata: Add copyright information

See https://blogs.gnome.org/hughsie/2013/10/01/copyright-in-appdata-files/ for details.
Status Job ID Name Coverage
  Test
passed #14517
run-appdata

00:00:05

passed #14516
run-tests

00:02:58

 
  Build
passed #14518
run-build

00:00:12