Add OSS headers

src/Compilers/Core/Portable/PEWriter/ComMemoryStream.cs

-using System;
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using System;
 using System.IO;
 using System.Collections.Generic;
 using System.Runtime.InteropServices.ComTypes;

src/Compilers/Core/Portable/PEWriter/IUnsafeComStream.cs

-using System;
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+using System;
 using System.Runtime.InteropServices;
 using System.Runtime.InteropServices.ComTypes;