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

src/libstd/lib.rs

@@ -273,6 +273,7 @@
 #![feature(macro_vis_matcher)]
 #![feature(needs_panic_runtime)]
 #![feature(never_type)]
+#![cfg_attr(not(stage0), feature(nll))]
 #![feature(exhaustive_patterns)]
 #![feature(on_unimplemented)]
 #![feature(optin_builtin_traits)]