constraint: Add a private header

And move the only private ClutterConstraint method to it.

This commit also sneaks in a change that makes sense for the debugging
of the update_allocation() method, which checks if the allocation was
effectively changed.

clutter/clutter-private.h

@@ -245,10 +245,6 @@ gboolean _clutter_boolean_continue_accumulator (GSignalInvocationHint *ihint,
 
 void _clutter_run_repaint_functions (ClutterRepaintFlags flags);
 
-void _clutter_constraint_update_allocation (ClutterConstraint *constraint,
-                                            ClutterActor      *actor,
-                                            ClutterActorBox   *allocation);
-
 GType _clutter_layout_manager_get_child_meta_type (ClutterLayoutManager *manager);
 
 void  _clutter_util_fully_transform_vertices (const CoglMatrix    *modelview,