New upstream git snapshot

* New upstream git snapshot with support for peer-to-peer software
  collections, required by Flatpak's peer-to-peer app sharing feature
  - d/copyright: Update
  - d/libostree-1-1.symbols: Update
  - Build-depend on Avahi libraries

debian/copyright

@@ -21,7 +21,7 @@ Copyright:
  © 2014 Owen Taylor <otaylor@redhat.com>
  © 2015 Dan Nicholson <nicholson@endlessm.com>
  © 2015 Canonical Ltd.
- © 2017 Endless Mobile, Inc.
+ © 2017-2018 Endless Mobile, Inc.
  © 2018 Matthew Leeds
 License: LGPL-2+