Remove dependency on .NET include file

[SamBent]

• .NET Core Version: (e.g. 3.0 Preview1, or daily build number, use dotnet --info) 5.0.0-alpha.1.19564.1
• Windows version: (winver) 1909 (OS Build 10.0.18363.476)
• Does the bug reproduce also in WPF for .NET Framework 4.8?: No
• Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? No

Problem description: Build failure when suitable .NET SDK is not installed.

Repro steps:

1. New machine with Win10 1909 (hence .NET 4.8).
2. Setup machine per WPF developer guide.
3. Fork/clone master branch of dotnet/wpf.
4. build

Actual behavior:
D:\repos\wpf\src\Microsoft.DotNet.Wpf\src\DirectWriteForwarder\CPP\DWriteWrapper\Common.h(15,10): fatal error C1083: Cannot open include file: 'CorError.h': No such file or directory [D:\repos\wpf\src\Microsoft.DotNet.Wpf\src\DirectWriteForwarder\DirectWriteForwarder.vcxproj]

Expected behavior: No build failure.

This failure goes away after installing .NET 4.6.1 SDK, which makes a version of CorError.h visible to
DirectWriteForwarder.vcxproj.