patch 8.2.1335: CTRL-C in the GUI doesn't interrupt

Problem:    CTRL-C in the GUI doesn't interrupt. (Sergey Vlasov)
Solution:   Recognize "C" with CTRL modifier as CTRL-C. (issue #6565)
diff --git a/src/proto/gui.pro b/src/proto/gui.pro
index 51bddf9..d762420 100644
--- a/src/proto/gui.pro
+++ b/src/proto/gui.pro
@@ -65,4 +65,5 @@
 char_u *get_find_dialog_text(char_u *arg, int *wwordp, int *mcasep);
 int gui_do_findrepl(int flags, char_u *find_text, char_u *repl_text, int down);
 void gui_handle_drop(int x, int y, int_u modifiers, char_u **fnames, int count);
+int check_for_interrupt(int key, int modifiers_arg);
 /* vim: set ft=c : */