Some projects such as GNOME-Software need this information to know
if its safe to build against (libsoup2 vs libsoup3 conflicts).
(cherry picked from commit f1dda39e80)
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>