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/proto/ex_getln.pro b/src/proto/ex_getln.pro
index 8845b9c..8095eff 100644
--- a/src/proto/ex_getln.pro
+++ b/src/proto/ex_getln.pro
@@ -1,14 +1,14 @@
 /* ex_getln.c */
 void cmdline_init(void);
-char_u *getcmdline(int firstc, long count, int indent);
+char_u *getcmdline(int firstc, long count, int indent, int do_concat);
 char_u *getcmdline_prompt(int firstc, char_u *prompt, int attr, int xp_context, char_u *xp_arg);
 int text_locked(void);
 void text_locked_msg(void);
 char *get_text_locked_msg(void);
 int curbuf_locked(void);
 int allbuf_locked(void);
-char_u *getexline(int c, void *cookie, int indent);
-char_u *getexmodeline(int promptc, void *cookie, int indent);
+char_u *getexline(int c, void *cookie, int indent, int do_concat);
+char_u *getexmodeline(int promptc, void *cookie, int indent, int do_concat);
 int cmdline_overstrike(void);
 int cmdline_at_end(void);
 colnr_T cmdline_getvcol_cursor(void);