patch 8.1.1494: build failure

Problem:    Build failure.
Solution:   Add missing changes.
diff --git a/src/structs.h b/src/structs.h
index c68989a..5f27d42 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1991,6 +1991,9 @@
     POPPOS_TOPRIGHT,
     POPPOS_CENTER
 } poppos_T;
+
+# define POPUPWIN_DEFAULT_ZINDEX    50
+# define POPUPMENU_ZINDEX	    100
 #endif
 
 /*