While flatpak carefully doesn’t expose any OSTree symbols in its C API,
it does sometimes return GErrors with the domain `OSTREE_GPG_ERROR`.
Applications can happily link against flatpak and receive such errors,
but won’t be able to understand them without also linking against
OSTree.
OSTree is a hard dependency of flatpak, so we might as well move it to
`Requires` rather than `Requires.private` to ensure that clients link
against it.
See https://gitlab.gnome.org/GNOME/gnome-software/merge_requests/336/diffs#note_650999
Signed-off-by: Philip Withnall <withnall@endlessm.com>
