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-bootconfig-parser.h

@@ -30,38 +30,47 @@ G_BEGIN_DECLS
 
 typedef struct _OstreeBootconfigParser OstreeBootconfigParser;
 
+_OSTREE_PUBLIC
 GType ostree_bootconfig_parser_get_type (void) G_GNUC_CONST;
 
+_OSTREE_PUBLIC
 OstreeBootconfigParser * ostree_bootconfig_parser_new (void);
 
+_OSTREE_PUBLIC
 OstreeBootconfigParser * ostree_bootconfig_parser_clone (OstreeBootconfigParser *self);
 
+_OSTREE_PUBLIC
 gboolean ostree_bootconfig_parser_parse (OstreeBootconfigParser  *self,
                                          GFile           *path,
                                          GCancellable    *cancellable,
                                          GError         **error);
 
+_OSTREE_PUBLIC
 gboolean ostree_bootconfig_parser_parse_at (OstreeBootconfigParser  *self,
                                             int                      dfd,
                                             const char              *path,
                                             GCancellable    *cancellable,
                                             GError         **error);
 
+_OSTREE_PUBLIC
 gboolean ostree_bootconfig_parser_write (OstreeBootconfigParser   *self,
                                          GFile            *output,
                                          GCancellable     *cancellable,
                                          GError          **error);
 
+_OSTREE_PUBLIC
 gboolean ostree_bootconfig_parser_write_at (OstreeBootconfigParser   *self,
                                             int                       dfd,
                                             const char               *path,
                                             GCancellable             *cancellable,
                                             GError                  **error);
 
+_OSTREE_PUBLIC
 void ostree_bootconfig_parser_set (OstreeBootconfigParser  *self,
                                    const char      *key,
                                    const char      *value);
 
+_OSTREE_PUBLIC
 const char *ostree_bootconfig_parser_get (OstreeBootconfigParser  *self,
                                           const char      *key);