Fix HD sub-shader serialization backwards compatibility

com.unity.render-pipelines.high-definition/HDRP/Editor/ShaderGraph/HDPBRSubShader.cs

 
 namespace UnityEditor.Experimental.Rendering.HDPipeline
 {
+    [FormerName("UnityEditor.ShaderGraph.HDPBRSubShader")]
     public class HDPBRSubShader : IPBRSubShader
     {
         Pass m_PassGBuffer = new Pass()