[K/N] Mark WeakReference with ExperimentalNativeApi

As a part of efforts to stabilize Native stdlib #KT-55765.

kotlin-native/backend.native/tests/runtime/basic/cleaner_workers.kt

@@ -2,7 +2,7 @@
  * Copyright 2010-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the LICENSE file.
  */
-@file:OptIn(ExperimentalStdlibApi::class, FreezingIsDeprecated::class)
+@file:OptIn(ExperimentalStdlibApi::class, FreezingIsDeprecated::class, kotlin.experimental.ExperimentalNativeApi::class)
 
 package runtime.basic.cleaner_workers