patch 9.1.0852: No warning when X11 registers are not available

Problem:  No warning when X11 registers are not available
          (delvh)
Solution: Output W23 once when connection to X11 clipboard/selection
          is not possible, mention in the documentation, that register 0
          will be used instead

Vim silently uses the 0 register, when clipboard or selection register * or +
are not available. This might be a bit unexpected for the user.

So let's just warn once when this happens.

fixes: #14768
closes: #16013

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/proto/message.pro b/src/proto/message.pro
index 6657a08..54a0a77 100644
--- a/src/proto/message.pro
+++ b/src/proto/message.pro
@@ -73,6 +73,7 @@
 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);
+void msg_warn_missing_clipboard(void);
 int do_dialog(int type, char_u *title, char_u *message, char_u *buttons, int dfltbutton, char_u *textfield, int ex_cmd);
 int vim_dialog_yesno(int type, char_u *title, char_u *message, int dflt);
 int vim_dialog_yesnocancel(int type, char_u *title, char_u *message, int dflt);