patch 8.1.1588: in :let-heredoc line continuation is recognized

Problem:    In :let-heredoc line continuation is recognized.
Solution:   Do not consume line continuation. (Ozaki Kiichi, closes #4580)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index faedda3..dde607c 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -3234,7 +3234,8 @@
 get_list_line(
     int	    c UNUSED,
     void    *cookie,
-    int	    indent UNUSED)
+    int	    indent UNUSED,
+    int	    do_concat UNUSED)
 {
     listitem_T **p = (listitem_T **)cookie;
     listitem_T *item = *p;