patch 9.0.1336: functions without arguments are not always declared properly

Problem:    Functions without arguments are not always declared properly.
Solution:   Use "(void)" instead of "()". (Yegappan Lakshmanan, closes #12031)

src/channel.c

@@ -4180,7 +4180,7 @@ channel_handle_events(int only_keep_open)
  * Return TRUE when there is any channel with a keep_open flag.
  */
     int
-channel_any_keep_open()
+channel_any_keep_open(void)
 {
     channel_T	*channel;