libgimpwidgets: fix GimpChainButton init().

So the minor fix on commit 3e35fe4d80 was not so minor after all, and I
should have looked more in details into its contents.

GET_PRIVATE() macro uses the `priv` pointer value hence had to be called
after gimp_chain_button_get_instance_private(). Thus the order was
important; the initialization and assignment values were actually
different. Simply let's get rid of the previous initialization of
`private` (make it declaration-only) to avoid cppcheck to (wrongfully)
complain about double variable assignment to the same value.

libgimpwidgets/gimpchainbutton.c

@@ -192,10 +192,11 @@ gimp_chain_button_class_init (GimpChainButtonClass *klass)
 static void
 gimp_chain_button_init (GimpChainButton *button)
 {
-  GimpChainButtonPrivate *private = GET_PRIVATE (button);
+  GimpChainButtonPrivate *private;
 
   button->priv = gimp_chain_button_get_instance_private (button);
 
+  private           = GET_PRIVATE (button);
   private->position = GIMP_CHAIN_TOP;
   private->active   = FALSE;
   private->image    = gtk_image_new ();