[gtk+] Add a comment

[Matthias Clasen <matthiasc src gnome org>]

commit 9629f9acc9b5e8d7c3ef0fb10e3273b2aeec49d1
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Nov 17 00:37:10 2017 -0500

    Add a comment
    
    Document that gdk-private.h is meant for private gdk<>gtk apis.

 gdk/gdk-private.h |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/gdk/gdk-private.h b/gdk/gdk-private.h
index 7f5daec..1455464 100644
--- a/gdk/gdk-private.h
+++ b/gdk/gdk-private.h
@@ -4,6 +4,8 @@
 #include <gdk/gdk.h>
 #include "gdk/gdkinternals.h"
 
+/* Private API for use in GTK+ */
+
 GdkDisplay *    gdk_display_open_default        (void);
 
 gboolean        gdk_device_grab_info            (GdkDisplay  *display,