[gtk/wip/jfelder/customsorter-null] customsorter: Add missing nullable annotation

[Jean Felder <jfelder src gnome org>]

commit b2fe1c00fd605b14277e21fbef2c26653112cc28
Author: Jean Felder <jean felder gmail com>
Date:   Sat Dec 5 00:00:57 2020 +0100

    customsorter: Add missing nullable annotation

 gtk/gtkcustomsorter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/gtkcustomsorter.c b/gtk/gtkcustomsorter.c
index cbc07a60de..87cb7b11bc 100644
--- a/gtk/gtkcustomsorter.c
+++ b/gtk/gtkcustomsorter.c
@@ -101,7 +101,7 @@ gtk_custom_sorter_init (GtkCustomSorter *self)
 
 /**
  * gtk_custom_sorter_new:
- * @sort_func: the #GCompareDataFunc to use for sorting
+ * @sort_func: (nullable): the #GCompareDataFunc to use for sorting
  * @user_data: (nullable): user data to pass to @sort_func
  * @user_destroy: (nullable): destroy notify for @user_data
  *