patch 8.0.0683: visual bell flashes too quickly

Problem:    When using a visual bell there is no delay, causing the flash to
            be very short, possibly unnoticeable.  Also, the flash and the
            beep can lockup the UI when repeated often.
Solution:   Do the delay in Vim or flush the output before the delay. Limit the
            bell to once per half a second. (Ozaki Kiichi, closes #1789)
diff --git a/src/proto/term.pro b/src/proto/term.pro
index 7d9fc6a..9af725f 100644
--- a/src/proto/term.pro
+++ b/src/proto/term.pro
@@ -16,6 +16,7 @@
 void out_trash(void);
 void out_char(unsigned c);
 void out_str_nf(char_u *s);
+void out_str_cf(char_u *s);
 void out_str(char_u *s);
 void term_windgoto(int row, int col);
 void term_cursor_right(int i);