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/popupmenu.c

@@ -387,7 +387,7 @@ pum_display(
  * This will avoid clearing and redrawing the popup menu, prevent flicker.
  */
     void
-pum_call_update_screen()
+pum_call_update_screen(void)
 {
     call_update_screen = TRUE;