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-diff.h

@@ -47,11 +47,15 @@ struct _OstreeDiffItem
   char *target_checksum;
 };
 
+_OSTREE_PUBLIC
 OstreeDiffItem *ostree_diff_item_ref (OstreeDiffItem *diffitem);
+_OSTREE_PUBLIC
 void ostree_diff_item_unref (OstreeDiffItem *diffitem);
 
+_OSTREE_PUBLIC
 GType ostree_diff_item_get_type (void);
 
+_OSTREE_PUBLIC
 gboolean ostree_diff_dirs (OstreeDiffFlags flags,
                            GFile          *a,
                            GFile          *b,
@@ -61,6 +65,7 @@ gboolean ostree_diff_dirs (OstreeDiffFlags flags,
                            GCancellable   *cancellable,
                            GError        **error);
 
+_OSTREE_PUBLIC
 void ostree_diff_print (GFile          *a,
                         GFile          *b,
                         GPtrArray      *modified,