Fill out an unstable implementation

This is verified to compile and work against the current version of
rust-lang/rust#40939.
diff --git a/src/lib.rs b/src/lib.rs
index d4b8b23..269cf73 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,5 @@
+#![cfg_attr(feature = "unstable", feature(proc_macro))]
+
 extern crate proc_macro;
 
 #[cfg(not(feature = "unstable"))]