Fix a compile-fail test

Author: oli-obk

src/test/compile-fail/const-fn-error.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(const_fn)]
+#![feature(const_fn, const_let)]
 
 const X : usize = 2;