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-repo-file.h

@@ -40,38 +40,52 @@ struct _OstreeRepoFileClass
   GObjectClass parent_class;
 };
 
+_OSTREE_PUBLIC
 GType   ostree_repo_file_get_type (void) G_GNUC_CONST;
 
+_OSTREE_PUBLIC
 gboolean ostree_repo_file_ensure_resolved (OstreeRepoFile  *self,
                                             GError         **error);
 
+_OSTREE_PUBLIC
 gboolean ostree_repo_file_get_xattrs (OstreeRepoFile  *self,
                                       GVariant       **out_xattrs,
                                       GCancellable    *cancellable,
                                       GError         **error);
 
+_OSTREE_PUBLIC
 OstreeRepo * ostree_repo_file_get_repo (OstreeRepoFile  *self);
+_OSTREE_PUBLIC
 OstreeRepoFile * ostree_repo_file_get_root (OstreeRepoFile  *self);
 
+_OSTREE_PUBLIC
 void ostree_repo_file_make_empty_tree (OstreeRepoFile  *self);
 
+_OSTREE_PUBLIC
 void ostree_repo_file_tree_set_metadata (OstreeRepoFile *self,
                                           const char     *checksum,
                                           GVariant       *metadata);
 
+_OSTREE_PUBLIC
 const char *ostree_repo_file_tree_get_contents_checksum (OstreeRepoFile  *self);
+_OSTREE_PUBLIC
 const char *ostree_repo_file_tree_get_metadata_checksum (OstreeRepoFile  *self);
 
+_OSTREE_PUBLIC
 GVariant *ostree_repo_file_tree_get_contents (OstreeRepoFile *self);
+_OSTREE_PUBLIC
 GVariant *ostree_repo_file_tree_get_metadata (OstreeRepoFile *self);
 
+_OSTREE_PUBLIC
 const char * ostree_repo_file_get_checksum (OstreeRepoFile  *self);
 
+_OSTREE_PUBLIC
 int     ostree_repo_file_tree_find_child  (OstreeRepoFile  *self,
                                             const char      *name,
                                             gboolean        *is_dir,
                                             GVariant       **out_container);
 
+_OSTREE_PUBLIC
 gboolean ostree_repo_file_tree_query_child (OstreeRepoFile  *self,
                                              int              n,
                                              const char      *attributes,