Added feature flag to use cell_update

Author: blitzerr

library/alloc/src/raw_vec/tests.rs

@@ -1,6 +1,10 @@
+#![feature(cell_update)]
+
 use super::*;
 use std::cell::Cell;
 
+
+
 #[test]
 fn allocator_param() {
     use crate::alloc::AllocErr;