fixed missing copyright header in EditorForegroundRunner.cs

Scripts/Editor/Utility/EditorForegroundRunner.cs

@@ -1,4 +1,11 @@
-using System;
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+using System;
 using System.Collections.Concurrent;
 using UnityEditor;