patch 8.2.3930: getcmdline() argument has a misleading type

Problem:    getcmdline() argument has a misleading type.
Solution:   Use the correct type, even though the value is not used.
diff --git a/src/proto/ex_getln.pro b/src/proto/ex_getln.pro
index d6f1911..d56893a 100644
--- a/src/proto/ex_getln.pro
+++ b/src/proto/ex_getln.pro
@@ -1,6 +1,6 @@
 /* ex_getln.c */
 void cmdline_init(void);
-char_u *getcmdline(int firstc, long count, int indent, int do_concat);
+char_u *getcmdline(int firstc, long count, int indent, getline_opt_T do_concat);
 char_u *getcmdline_prompt(int firstc, char_u *prompt, int attr, int xp_context, char_u *xp_arg);
 int check_opt_wim(void);
 int text_and_win_locked(void);