Link to rustc_driver crate in plugin

This is in anticipation for rust-lang/rust#56987 where the
`rustc_driver` crate being linked in will be required to link correctly
against the compiler. In the meantime it should be harmless otherwise!

src/lib.rs

@@ -17,6 +17,8 @@
 // (currently there is no way to opt into sysroot crates w/o `extern crate`)
 #[allow(unused_extern_crates)]
 extern crate rustc_plugin;
+#[allow(unused_extern_crates)]
+extern crate rustc_driver;
 use self::rustc_plugin::Registry;
 
 #[plugin_registrar]