[gtk+] Drop a leftover declaration

[Matthias Clasen <matthiasc src gnome org>]

commit 1a9a0c25771d124a54b8bf7d81e36033ba429970
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Feb 5 20:34:03 2018 +0100

    Drop a leftover declaration
    
    The function gdk_add_option_entries does not exist anymore.

 gdk/gdk-private.h |    2 --
 1 files changed, 0 insertions(+), 2 deletions(-)
---
diff --git a/gdk/gdk-private.h b/gdk/gdk-private.h
index d262809..7c4cd30 100644
--- a/gdk/gdk-private.h
+++ b/gdk/gdk-private.h
@@ -13,8 +13,6 @@ gboolean        gdk_device_grab_info            (GdkDisplay  *display,
                                                  GdkWindow  **grab_window,
                                                  gboolean    *owner_events);
 
-void            gdk_add_option_entries          (GOptionGroup *group);
-
 void            gdk_pre_parse                   (void);
 
 void            gdk_window_freeze_toplevel_updates      (GdkWindow *window);