Add warn(unreachable_pub) to rustc_graphviz.

Author: nnethercote

compiler/rustc_graphviz/src/lib.rs

@@ -277,6 +277,7 @@
 )]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::borrow::Cow;