[K/N] Mark WeakReference with ExperimentalNativeApi

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

kotlin-native/runtime/src/main/kotlin/kotlin/native/ref/Weak.kt

@@ -5,11 +5,14 @@
 
 package kotlin.native.ref
 
+import kotlin.experimental.ExperimentalNativeApi
+
 /**
  * Class WeakReference encapsulates weak reference to an object, which could be used to either
  * retrieve a strong reference to an object, or return null, if object was already destroyed by
  * the memory manager.
  */
+@ExperimentalNativeApi
 public class WeakReference<T : Any> {
     /**
      * Creates a weak reference object pointing to an object. Weak reference doesn't prevent