add cfg_attr to use bevy_reflect::Reflect

Author: 2ne1ugly

crates/bevy_ecs/src/entity/index_set.rs

@@ -21,6 +21,8 @@ use indexmap::set::{self, IndexSet};
 use super::{Entity, EntityHash, EntitySetIterator};
 
 use bevy_platform::prelude::Box;
+
+#[cfg_attr(feature = "bevy_reflect", derive(Reflect))]
 use bevy_reflect::Reflect;
 
 /// An [`IndexSet`] pre-configured to use [`EntityHash`] hashing.