bench: libcore: fix build failure of any.rs benchmark (use "dyn Any")

Author: matthiaskrgr

src/libcore/benches/any.rs

@@ -15,7 +15,7 @@ use test::{Bencher, black_box};
 fn bench_downcast_ref(b: &mut Bencher) {
     b.iter(|| {
         let mut x = 0;
-        let mut y = &mut x as &mut Any;
+        let mut y = &mut x as &mut dyn Any;
         black_box(&mut y);
         black_box(y.downcast_ref::<isize>() == Some(&0));
     });