patch 8.2.3149: some plugins have a problem with the error check

Problem:    Some plugins have a problem with the error check for using
            :command with -complete but without -nargs.
Solution:   In legacy script only give a warning message.
diff --git a/src/proto/message.pro b/src/proto/message.pro
index 16c3354..d3ec949 100644
--- a/src/proto/message.pro
+++ b/src/proto/message.pro
@@ -69,6 +69,7 @@
 void verbose_stop(void);
 int verbose_open(void);
 void give_warning(char_u *message, int hl);
+void give_warning_with_source(char_u *message, int hl, int with_source);
 void give_warning2(char_u *message, char_u *a1, int hl);
 void msg_advance(int col);
 int do_dialog(int type, char_u *title, char_u *message, char_u *buttons, int dfltbutton, char_u *textfield, int ex_cmd);