Fix type does not support direct content in XAML

Add reference to UIAutomationProvider.
https://stackoverflow.com/a/35580095/121348

Author: jcansdale

src/GitHub.VisualStudio.UI/GitHub.VisualStudio.UI.csproj

@@ -13,6 +13,7 @@
     <Reference Include="System.Xaml" />
     <Reference Include="PresentationCore" />
     <Reference Include="PresentationFramework" />
+    <Reference Include="UIAutomationProvider" />
     <Reference Include="WindowsBase" />
   </ItemGroup>