patch 9.0.1867: Vim9: access to interface statics possible

Problem:  Vim9: access to interface statics possible
Solution: Prevent direct access to interface statics

closes: #13007

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ernie Rael <errael@raelity.com>
diff --git a/src/vim9.h b/src/vim9.h
index cf24c8d..e30ebda 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -498,6 +498,7 @@
 typedef struct {
     class_T	*cm_class;
     int		cm_idx;
+    int		cm_static;
 } classmember_T;
 // arguments to ISN_STOREINDEX
 typedef struct {