More ISO C90 warning fixes in Clutter

This time, in Clutter core.

The ObjC standard library provides a type called 'id', which obviously
requires any library to either drop the useful shadowed variable warning
or stop using 'id' as a variable name.

Yes, it's almost unbearably stupid. Well, at least it's not 'index' in
string.h, or 'y2' in math.h.

clutter/clutter-timeout-pool.h

@@ -60,7 +60,7 @@ guint               clutter_timeout_pool_add    (ClutterTimeoutPool *pool,
                                                  gpointer            data,
                                                  GDestroyNotify      notify);
 void                clutter_timeout_pool_remove (ClutterTimeoutPool *pool,
-                                                 guint               id);
+                                                 guint               id_);
 
 #endif /* CLUTTER_DISABLE_DEPRECATED */