Fix hash-stable-is-unstable test

Author: gnzlbg

src/test/ui-fulldeps/hash-stable-is-unstable.rs

@@ -12,4 +12,5 @@ use rustc_macros::HashStable;
 
 #[derive(HashStable)]
 //~^ use of unstable library feature 'rustc_private'
+//~| use of unstable library feature 'rustc_private'
 struct Test;