fix tidy link

Author: yoshuawuyts

library/coretests/tests/mem.rs

@@ -1,8 +1,8 @@
 use core::mem::*;
 use core::{array, ptr};
+use std::cell::Cell;
 #[cfg(panic = "unwind")]
 use std::rc::Rc;
-use std::cell::Cell;
 
 #[test]
 fn size_of_basic() {