patch 8.1.1600: cannot specify highlighting for popup window scrollbar

Problem:    Cannot specify highlighting for popup window scrollbar.
Solution:   Add "scrollbarhighlight" and "thumbhighlight" options.
diff --git a/src/structs.h b/src/structs.h
index 9992e99..b368c9e 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2904,6 +2904,8 @@
     int		w_firstline;	    // "firstline" for popup window
     int		w_want_scrollbar;   // when zero don't use a scrollbar
     int		w_has_scrollbar;    // scrollbar displayed
+    char_u	*w_scrollbar_highlight; // "scrollbarhighlight"
+    char_u	*w_thumb_highlight; // "thumbhighlight"
     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