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-gpg-verify-result.h

@@ -82,21 +82,27 @@ typedef enum {
   OSTREE_GPG_SIGNATURE_ATTR_USER_EMAIL
 } OstreeGpgSignatureAttr;
 
+_OSTREE_PUBLIC
 GType ostree_gpg_verify_result_get_type (void);
 
+_OSTREE_PUBLIC
 guint ostree_gpg_verify_result_count_all (OstreeGpgVerifyResult *result);
 
+_OSTREE_PUBLIC
 guint ostree_gpg_verify_result_count_valid (OstreeGpgVerifyResult *result);
 
+_OSTREE_PUBLIC
 gboolean ostree_gpg_verify_result_lookup (OstreeGpgVerifyResult *result,
                                           const gchar *key_id,
                                           guint *out_signature_index);
 
+_OSTREE_PUBLIC
 GVariant * ostree_gpg_verify_result_get (OstreeGpgVerifyResult *result,
                                          guint signature_index,
                                          OstreeGpgSignatureAttr *attrs,
                                          guint n_attrs);
 
+_OSTREE_PUBLIC
 GVariant * ostree_gpg_verify_result_get_all (OstreeGpgVerifyResult *result,
                                              guint signature_index);
 
@@ -113,12 +119,14 @@ typedef enum {
   OSTREE_GPG_SIGNATURE_FORMAT_DEFAULT = 0
 } OstreeGpgSignatureFormatFlags;
 
+_OSTREE_PUBLIC
 void ostree_gpg_verify_result_describe (OstreeGpgVerifyResult *result,
                                         guint signature_index,
                                         GString *output_buffer,
                                         const gchar *line_prefix,
                                         OstreeGpgSignatureFormatFlags flags);
 
+_OSTREE_PUBLIC
 void ostree_gpg_verify_result_describe_variant (GVariant *variant,
                                                 GString *output_buffer,
                                                 const gchar *line_prefix,