Put use of `UnityEditor` in `UNITY_EDITOR` so that builds don't fail

ScriptableRenderPipeline/LightweightPipeline/Data/LightweightPipelineAsset.cs

+#if UNITY_EDITOR
+#endif
 
 namespace UnityEngine.Experimental.Rendering.LightweightPipeline
 {
                 AssetDatabase.CreateAsset(instance, pathName);
             }
         }
-
 #endif
 
         protected override IRenderPipeline InternalCreatePipeline()