enable editing of multiple decal projectors

com.unity.render-pipelines.high-definition/HDRP/Material/Decal/DecalProjectorComponent.cs

 namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
     [ExecuteInEditMode]
+    [CanEditMultipleObjects]
     public class DecalProjectorComponent : MonoBehaviour
     {
         public Material m_Material = null;