enable question_mark feature in rustc_borrowck

Author: Jorge Aparicio

src/librustc_borrowck/lib.rs

@@ -25,6 +25,7 @@
 #![feature(staged_api)]
 #![feature(associated_consts)]
 #![feature(nonzero)]
+#![feature(question_mark)]
 #[macro_use] extern crate log;
 #[macro_use] extern crate syntax;