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/ex_cmds.h b/src/ex_cmds.h
index 4e5c0aa..647dce0 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -1837,7 +1837,7 @@
     int		bad_char;	/* BAD_KEEP, BAD_DROP or replacement byte */
     int		useridx;	/* user command index */
     char	*errmsg;	/* returned error message */
-    char_u	*(*getline)(int, void *, int);
+    char_u	*(*getline)(int, void *, int, int);
     void	*cookie;	/* argument for getline() */
 #ifdef FEAT_EVAL
     struct condstack *cstack;	/* condition stack for ":if" etc. */