Native: prepare existing tests for cinterop adding ExperimentalForeignApi

cinterop tool should add ExperimentalForeignApi to all generated
declarations by default. This commit prepares existing tests for this.

^KT-58362

kotlin-native/backend.native/tests/interop/objc/kt55938/lib.kt

@@ -2,6 +2,7 @@
  * Copyright 2010-2023 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
+@file:OptIn(kotlinx.cinterop.ExperimentalForeignApi::class)
 
 package kt55938lib