fix unused import error

Author: Jorge Aparicio

src/librustc/middle/region.rs

@@ -22,6 +22,8 @@ use util::nodemap::{FnvHashMap, FnvHashSet, NodeMap};
 use util::common::can_reach;
 
 use std::cell::RefCell;
+// NOTE(stage0) remove import after a snapshot
+#[cfg(stage0)]
 use std::hash::{Hash};
 use syntax::codemap::Span;
 use syntax::{ast, visit};