Fix GH-17891 gh17373.phpt test issue without freetype support

skip if imagefttext() is not available

This test calls imagefttext(), which may not be available if libgd
was built without freetype support.

Closes GH-17910

ext/gd/tests/gh17373.phpt

@@ -2,6 +2,10 @@
 Bug GH-17373 (imagefttext() ignores clipping rect for palette images)
 --EXTENSIONS--
 gd
+--SKIPIF--
+<?php
+if(!function_exists('imagefttext')) die('skip imagefttext() not available');
+?>
 --FILE--
 <?php
 $im = imagecreate(64, 32);