Add .NET Framework ref assemblies to enable linux build (#625)

Fixes #421

Terminal.Gui/Terminal.Gui.csproj

@@ -169,6 +169,7 @@
     <DebugType></DebugType>
   </PropertyGroup>
   <ItemGroup>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="true" />
     <PackageReference Include="NStack.Core" Version="0.14.0" />
   </ItemGroup>
   <ItemGroup>