quartz: make setting_same_owner member of GtkClipboardOwner @public

to fix the build, thanks to parafin for the patch.
(cherry picked from commit e3e055f855)

gtk/gtkclipboard-quartz.c

@@ -40,6 +40,7 @@ enum {
 
 @interface GtkClipboardOwner : NSObject {
   GtkClipboard *clipboard;
+  @public
   gboolean setting_same_owner;
 }