[gtk/misc-multiselection] Add a forgotten export

[Matthias Clasen <matthiasc src gnome org>]

commit b0cf7a95b061d92de394f3e1410e81bf77d8082f
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Jun 6 11:21:05 2020 -0400

    Add a forgotten export
    
    gtk_selection_model_unselect_callback needs to be exported.

 gtk/gtkselectionmodel.h | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/gtk/gtkselectionmodel.h b/gtk/gtkselectionmodel.h
index 2a6318d6f2..86657fba15 100644
--- a/gtk/gtkselectionmodel.h
+++ b/gtk/gtkselectionmodel.h
@@ -51,6 +51,10 @@ G_DECLARE_INTERFACE (GtkSelectionModel, gtk_selection_model, GTK, SELECTION_MODE
  * be changed, in which case @selected is set to %FALSE.
  *
  * @start_range and @n_items are set to return the range.
+ *
+ * The callback will be called repeatedly to find all ranges
+ * to operate on until it has exhausted the items of the model,
+ * or until it returns an empty range (ie @n_items == 0).
  */
 typedef void     (* GtkSelectionCallback) (guint     position,
                                            guint    *start_range,
@@ -147,6 +151,7 @@ GDK_AVAILABLE_IN_ALL
 gboolean                gtk_selection_model_select_callback     (GtkSelectionModel      *model,
                                                                  GtkSelectionCallback    callback,
                                                                  gpointer                data);
+GDK_AVAILABLE_IN_ALL
 gboolean                gtk_selection_model_unselect_callback   (GtkSelectionModel      *model,
                                                                  GtkSelectionCallback    callback,
                                                                  gpointer                data);