Make a common HDProbe ancestor

com.unity.render-pipelines.high-definition/HDRP/Lighting/Reflection/HDAdditionalReflectionData.cs

 namespace UnityEngine.Experimental.Rendering
 {
     [RequireComponent(typeof(ReflectionProbe))]
-    public class HDAdditionalReflectionData : MonoBehaviour, ISerializationCallbackReceiver
+    public class HDAdditionalReflectionData : HDProbe, ISerializationCallbackReceiver
     {
         [HideInInspector]
         const int currentVersion = 1;

com.unity.render-pipelines.high-definition/HDRP/Lighting/Reflection/PlanarReflectionProbe.cs

 namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
     [ExecuteInEditMode]
-    public class PlanarReflectionProbe : MonoBehaviour, ISerializationCallbackReceiver
+    public class PlanarReflectionProbe : HDProbe, ISerializationCallbackReceiver
     {
         [HideInInspector]
         const int currentVersion = 1;

com.unity.render-pipelines.high-definition/HDRP/Lighting/Reflection/HDProbe.cs

+using UnityEngine;
+
+namespace UnityEngine.Experimental.Rendering.HDPipeline
+{
+    public abstract class HDProbe : MonoBehaviour
+    {
+
+    }
+}

com.unity.render-pipelines.high-definition/HDRP/Lighting/Reflection/HDProbe.cs.meta

+fileFormatVersion: 2
+guid: 843017cb1cb3fa441a28e5a674bfea8e
+MonoImporter:
+  externalObjects: {}
+  serializedVersion: 2
+  defaultReferences: []
+  executionOrder: 0
+  icon: {instanceID: 0}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: