patch 8.1.1589: popup window does not indicate scroll position

Problem:    Popup window does not indicate scroll position.
Solution:   Add a scrollbar.
diff --git a/src/structs.h b/src/structs.h
index c664f0e..9992e99 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2902,6 +2902,8 @@
     int		w_wantline;	    // "line" for popup window
     int		w_wantcol;	    // "col" for popup window
     int		w_firstline;	    // "firstline" for popup window
+    int		w_want_scrollbar;   // when zero don't use a scrollbar
+    int		w_has_scrollbar;    // scrollbar displayed
     int		w_popup_padding[4]; // popup padding top/right/bot/left
     int		w_popup_border[4];  // popup border top/right/bot/left
     char_u	*w_border_highlight[4];  // popup border highlight