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/vim9compile.c b/src/vim9compile.c
index e2e5b76..d45dd63 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -291,7 +291,7 @@
  * Return TRUE if the script context is Vim9 script.
  */
     int
-script_is_vim9()
+script_is_vim9(void)
 {
     return SCRIPT_ITEM(current_sctx.sc_sid)->sn_version == SCRIPT_VERSION_VIM9;
 }