Fix kotlin-native/backend.native/tests/samples after 82524fd

Enable language feature for these tests, because some code relies
on implicit integer coercion there

kotlin-native/backend.native/tests/samples/build.gradle.kts

@@ -19,6 +19,10 @@ allprojects {
         val kotlinCompilerRepo: String? by rootProject
         kotlinCompilerRepo?.let { maven(it) }
     }
+
+    tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinNativeCompile>().configureEach {
+        compilerOptions.freeCompilerArgs.add("-XXLanguage:+ImplicitSignedToUnsignedIntegerConversion")
+    }
 }
 
 val hostOs = System.getProperty("os.name")