patch 8.2.2524: cannot change the characters displayed in the foldcolumn

Problem:    Cannot change the characters displayed in the foldcolumn.
Solution:   Add fields to 'fillchars'. (Yegappan Lakshmanan, Matthieu Coudron,
            closes #7860)
diff --git a/src/globals.h b/src/globals.h
index bc4a3d5..0fd08b6 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1347,6 +1347,9 @@
 EXTERN int	fill_stlnc INIT(= ' ');
 EXTERN int	fill_vert INIT(= ' ');
 EXTERN int	fill_fold INIT(= '-');
+EXTERN int	fill_foldopen INIT(= '-');
+EXTERN int	fill_foldclosed INIT(= '+');
+EXTERN int	fill_foldsep INIT(= '|');
 EXTERN int	fill_diff INIT(= '-');
 EXTERN int	fill_eob INIT(= '~');