patch 8.2.2454: leading space can not be made visible

Problem:    Leading space can not be made visible.
Solution:   Add "lead:" to 'listchars'. (closes #7772)
diff --git a/src/globals.h b/src/globals.h
index e601fd6..4daa092 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1352,6 +1352,7 @@
 EXTERN int	lcs_tab2 INIT(= NUL);
 EXTERN int	lcs_tab3 INIT(= NUL);
 EXTERN int	lcs_trail INIT(= NUL);
+EXTERN int	lcs_lead INIT(= NUL);
 #ifdef FEAT_CONCEAL
 EXTERN int	lcs_conceal INIT(= ' ');
 #endif