impl Unpin for UnparkEvents with feature flag

`UnparkEvents` is deprecated, but is still used in some cases.

Author: carllerche

src/task_impl/std/mod.rs

@@ -703,8 +703,9 @@ impl<T> From<Arc<T>> for NotifyHandle
 
 #[cfg(feature = "nightly")]
 mod nightly {
-    use super::TaskUnpark;
+    use super::{TaskUnpark, UnparkEvents};
     use core::marker::Unpin;
 
     impl Unpin for TaskUnpark {}
+    impl Unpin for UnparkEvents {}
 }