Add missing #![feature(min_const_fn)] to the newtype_index test.

Author: eddyb

src/test/run-pass-fulldeps/newtype_index.rs

@@ -1,4 +1,4 @@
-#![feature(rustc_attrs, step_trait, rustc_private)]
+#![feature(min_const_fn, rustc_attrs, rustc_private, step_trait)]
 
 #[macro_use] extern crate rustc_data_structures;
 extern crate rustc_serialize;