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)
diff --git a/src/channel.c b/src/channel.c
index ee5b512..33605d4 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -4180,7 +4180,7 @@
  * 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;