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/clipboard.c

@@ -257,7 +257,7 @@ start_global_changes(void)
  * right text.
  */
     static int
-is_clipboard_needs_update()
+is_clipboard_needs_update(void)
 {
     return clipboard_needs_update;
 }