[gradle-plugin] Fix MPP in the new plugin after switching to 1.2.50

tools/kotlin-native-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/experimental/plugins/KotlinPlatformNativePlugin.kt

@@ -25,4 +25,6 @@ class KotlinPlatformNativePlugin : KotlinPlatformImplementationPluginBase("nativ
             """.trimIndent())
         }
     }
+
+    override fun namedSourceSetsContainer(project: Project) = project.kotlinNativeSourceSets
 }