Omit empty derive attribute

Previously an empty #[derive()] would appear distractingly in
macro-expanded code.
diff --git a/macro/src/lib.rs b/macro/src/lib.rs
index fae874a..291b03c 100644
--- a/macro/src/lib.rs
+++ b/macro/src/lib.rs
@@ -9,6 +9,7 @@
 
 extern crate proc_macro;
 
+mod derive;
 mod expand;
 mod syntax;
 mod type_id;