Make it possible to build the style_traits crate with a stable compiler.

Testing this on CI to make sure we don’t regress it is blocked on #11806

components/style_traits/cursor.rs

@@ -8,7 +8,9 @@ use cssparser::ToCss;
 
 macro_rules! define_cursor {
     ($( $css: expr => $variant: ident = $value: expr, )+) => {
-        #[derive(Clone, Copy, Debug, Deserialize, Eq, HeapSizeOf, PartialEq, Serialize)]
+        #[derive(Clone, Copy, Debug, Eq, PartialEq)]
+        #[cfg_attr(feature = "serde-serialization", derive(Deserialize, Serialize))]
+        #[cfg_attr(feature = "heap_size", derive(HeapSizeOf))]
         #[repr(u8)]
         pub enum Cursor {
             $( $variant = $value ),+