Add missing SKIPIF for new ext/ffi test

Author: bwoebi

ext/ffi/tests/gh8433.phpt

@@ -1,5 +1,9 @@
 --TEST--
 GH-8433 (Assigning function pointers to structs in FFI leaks memory)
+--SKIPIF--
+<?php
+if (!extension_loaded("ffi")) die("skip ffi extension not available");
+?>
 --FILE--
 <?php