Add missing extern crate rustc_driver

Author: bjorn3

src/lib.rs

@@ -24,6 +24,11 @@ extern crate rustc_parse;
 extern crate rustc_session;
 extern crate rustc_span;
 
+// Necessary to pull in object code as the rest of the rustc crates are shipped only as rmeta
+// files.
+#[allow(unused_extern_crates)]
+extern crate rustc_driver;
+
 use std::cell::RefCell;
 use std::collections::HashMap;
 use std::fmt;