Fix linux build failure on reference assemblies (#245)

- Closes #244

Terminal.Gui/Terminal.Gui.csproj

@@ -60,6 +60,10 @@
   <ItemGroup>
     <PackageReference Include="NStack.Core" Version="0.11.0" />
     <PackageReference Include="System.ValueTuple" Version="4.4.0" />
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
   <ItemGroup>
   </ItemGroup>