docs: mark base interface as private

gtk/deprecated/gtkactivatable.h

@@ -54,8 +54,11 @@ typedef struct _GtkActivatableIface GtkActivatableIface;
 
 struct _GtkActivatableIface
 {
+  /*< private >*/
   GTypeInterface g_iface;
 
+  /*< public >*/
+
   /* virtual table */
   void   (* update)                   (GtkActivatable *activatable,
 		                       GtkAction      *action,

gtk/gtkactionable.h

@@ -38,8 +38,11 @@ typedef struct _GtkActionable                               GtkActionable;
 
 struct _GtkActionableInterface
 {
+  /*< private >*/
   GTypeInterface g_iface;
 
+  /*< public >*/
+
   const gchar * (* get_action_name)             (GtkActionable *actionable);
   void          (* set_action_name)             (GtkActionable *actionable,
                                                  const gchar   *action_name);

gtk/gtkstyleprovider.h

@@ -92,8 +92,11 @@ typedef struct _GtkStyleProvider GtkStyleProvider; /* dummy typedef */
  */
 struct _GtkStyleProviderIface
 {
+  /*< private >*/
   GTypeInterface g_iface;
 
+  /*< public >*/
+
   GtkStyleProperties * (* get_style) (GtkStyleProvider *provider,
                                       GtkWidgetPath    *path);