layers: Fix memleak of instance object in object tracker layer

CreateInstance calls CreateObject for the instance. There was no
corresponding DestroyObject in DestroyInstance.
diff --git a/layers/object_tracker_utils.cpp b/layers/object_tracker_utils.cpp
index d8d86c0..cb617d0 100644
--- a/layers/object_tracker_utils.cpp
+++ b/layers/object_tracker_utils.cpp
@@ -402,6 +402,8 @@
         instance_data->logging_callback.pop_back();
     }
 
+    DestroyObject(instance, instance, kVulkanObjectTypeInstance, pAllocator, VALIDATION_ERROR_258004ec, VALIDATION_ERROR_258004ee);
+
     layer_debug_utils_destroy_instance(instance_data->report_data);
     FreeLayerDataPtr(key, layer_data_map);