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/clipboard.c b/src/clipboard.c
index b09c678..4ce536a 100644
--- a/src/clipboard.c
+++ b/src/clipboard.c
@@ -257,7 +257,7 @@
  * right text.
  */
     static int
-is_clipboard_needs_update()
+is_clipboard_needs_update(void)
 {
     return clipboard_needs_update;
 }