Fix reference to screenspace shadow shader so that it can be referenced in a build.

ScriptableRenderPipeline/LightweightPipeline/LWRP/Data/LightweightPipelineAsset.cs

 
 
 #if UNITY_EDITOR
+        [NonSerialized]
         private LightweightPipelineEditorResources m_EditorResourcesAsset;
 
         [MenuItem("Assets/Create/Render Pipeline/Lightweight/Pipeline Asset", priority = CoreUtils.assetCreateMenuPriority1)]
         public Shader CopyDepthShader
         {
             get { return resources != null ? resources.CopyDepthShader : null; }
+        }
+        public Shader ScreenSpaceShadowShader
+        {
+            get { return resources != null ? resources.ScreenSpaceShadowShader : null; }
         }
     }
 }

ScriptableRenderPipeline/LightweightPipeline/LWRP/Data/LightweightPipelineResources.asset

   m_EditorClassIdentifier: 
   BlitShader: {fileID: 4800000, guid: c17132b1f77d20942aa75f8429c0f8bc, type: 3}
   CopyDepthShader: {fileID: 4800000, guid: d6dae50ee9e1bfa4db75f19f99355220, type: 3}
+  ScreenSpaceShadowShader: {fileID: 4800000, guid: 0f854b35a0cf61a429bd5dcfea30eddd,
+    type: 3}

ScriptableRenderPipeline/LightweightPipeline/LWRP/Data/LightweightPipelineResources.cs

 {
     public Shader BlitShader;
     public Shader CopyDepthShader;
+    public Shader ScreenSpaceShadowShader;
 }

ScriptableRenderPipeline/LightweightPipeline/LWRP/LightweightPipeline.cs

             m_BlitQuad = LightweightUtils.CreateQuadMesh(false);
             m_BlitMaterial = CoreUtils.CreateEngineMaterial(m_Asset.BlitShader);
             m_CopyDepthMaterial = CoreUtils.CreateEngineMaterial(m_Asset.CopyDepthShader);
+            m_ScreenSpaceShadowsMaterial = CoreUtils.CreateEngineMaterial(m_Asset.ScreenSpaceShadowShader);
-            m_ScreenSpaceShadowsMaterial = CoreUtils.CreateEngineMaterial("Hidden/LightweightPipeline/ScreenSpaceShadows");
         }
 
         public override void Dispose()