Mark sinary_trees_parallel::Tree with collector_id(SimpleCollectorId)

Fixes a compilation error

Author: Techcable

libs/simple/examples/binary_trees_parallel.rs

@@ -2,13 +2,14 @@
     arbitrary_self_types, // Unfortunately this is required for methods on Gc refs
 )]
 use zerogc::prelude::*;
-use zerogc_simple::{SimpleCollector, SimpleCollectorContext, Gc};
+use zerogc_simple::{SimpleCollector, SimpleCollectorContext, Gc, CollectorId as SimpleCollectorId};
 use zerogc_derive::Trace;
 
 use rayon::prelude::*;
 use slog::{Logger, Drain, o};
 
 #[derive(Trace)]
+#[zerogc(collector_id(SimpleCollectorId))]
 struct Tree<'gc> {
     #[zerogc(mutable(public))]
     children: GcCell<Option<(Gc<'gc, Tree<'gc>>, Gc<'gc, Tree<'gc>>)>>,