Don't rely on AS_BUNDLE_KIND_FLATPAK existing
Don't rely on AS_BUNDLE_KIND_FLATPAK existing

The appstream-glib in Ubuntu 16.04 didn't have this.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@debian.org>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak