lib: Introduce versioned symbols

As rpm-ostree evolves, it keeps driving API additions to libostree.
This creates a relatively tight coupling.

However, if delivering via e.g. RPM, unless one manually remembers to
increment the `Requires:` in the spec file, it's possible for the two
to become desynchronized.

RPM handles versioned symbols and will ensure a dependency if the
application starts using a newer version.

To implement this, switch to `-fvisibility=hidden`, along with an
annotation in the header, and finally add a `.sym` file.

This matches what other projects like systemd and libvirt do.

Although rather than attempting to retroactively version symbols, glom
them all onto the current one.

src/libostree/ostree-cmdprivate.h

@@ -29,7 +29,8 @@ typedef struct {
   gboolean (* ostree_static_delta_dump) (OstreeRepo *repo, const char *delta_id, GCancellable *cancellable, GError **error);
 } OstreeCmdPrivateVTable;
 
-const OstreeCmdPrivateVTable *
+/* Note this not really "public", we just export the symbol, but not the header */
+_OSTREE_PUBLIC const OstreeCmdPrivateVTable *
 ostree_cmd__private__ (void);
 
 G_END_DECLS