mark sys_common::once::generic::Once::new const-stable

library/std/src/sys_common/once/generic.rs

@@ -107,6 +107,7 @@ struct WaiterQueue<'a> {
 
 impl Once {
     #[inline]
+    #[rustc_const_stable(feature = "const_once_new", since = "1.32.0")]
     pub const fn new() -> Once {
         Once { state_and_queue: AtomicPtr::new(ptr::invalid_mut(INCOMPLETE)) }
     }