add missing feature flag

RalfJung · RalfJung · commit 1546e07e63b7 · 2019-02-14T16:20:58.000+01:00
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
@@ -124,6 +124,7 @@
 #![feature(adx_target_feature)]
 #![feature(maybe_uninit)]
 #![feature(unrestricted_attribute_tokens)]
+#![feature(external_doc)]
 
 #[prelude_import]
 #[allow(unused)]
