Return an error when animating column-count: auto

This changes the behaviour of animations of `column-count`
(hello Captain Obvious) but this is a bugfix.

components/style/values/generics/column.rs

@@ -11,6 +11,7 @@ pub enum ColumnCount<PositiveInteger> {
     /// A positive integer.
     Integer(PositiveInteger),
     /// The keyword `auto`.
+    #[animation(error)]
     Auto,
 }