fix bevy_hierarchy crate optional feature bevy_app (#12943)

fix bevy_hierarchy crate optional feature `bevy_app`

Author: ycysdf

crates/bevy_hierarchy/src/lib.rs

@@ -88,9 +88,11 @@ use bevy_app::prelude::*;
 /// Check the [crate-level documentation] for all the features.
 ///
 /// [crate-level documentation]: crate
+#[cfg(feature = "bevy_app")]
 #[derive(Default)]
 pub struct HierarchyPlugin;
 
+#[cfg(feature = "bevy_app")]
 impl Plugin for HierarchyPlugin {
     fn build(&self, app: &mut App) {
         app.register_type::<Children>()