Expose the Lit material UI to code outside SRP.

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Material/Lit/LitUI.cs

 
 namespace UnityEditor.Experimental.Rendering.HDPipeline
 {
-    class LitGUI : BaseLitGUI
+    public class LitGUI : BaseLitGUI
     {
         protected static class Styles
         {