[nll] libserialize: enable feature(nll) for bootstrap

src/libserialize/lib.rs

@@ -24,6 +24,7 @@ Core encoding and decoding interfaces.
 #![feature(core_intrinsics)]
 #![feature(specialization)]
 #![feature(never_type)]
+#![cfg_attr(not(stage0), feature(nll))]
 #![cfg_attr(test, feature(test))]
 
 pub use self::serialize::{Decoder, Encoder, Decodable, Encodable};