patch 9.0.1184: interface of an object is not recognized when checking type

Problem:    Interface of an object is not recognized when checking type.
Solution:   Use the interface implemented by an object.
diff --git a/src/structs.h b/src/structs.h
index 89ed23d..80dc473 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1500,6 +1500,7 @@
     // interfaces declared for the class
     int		class_interface_count;
     char_u	**class_interfaces;	// allocated array of names
+    class_T	**class_interfaces_cl;	// interfaces (counts as reference)
 
     // class members: "static varname"
     int		class_class_member_count;