i18n: KnowledgeBase JS

In 5120, the reporting party also mentioned that they receive an error of
`script '/xxx/osTicket/kb/ajax.php' not found or unable to stat`.  This is
due to the AJAX call being inside the KB directory, and `/kb/ajax.php` truly
does not exist. This adds `ROOT_PATH` to the beginning of the AJAX call so
that no matter where it is, it will point to the correct location.

include/client/footer.inc.php

@@ -12,7 +12,7 @@
 </div>
 <?php
 if (($lang = Internationalization::getCurrentLanguage()) && $lang != 'en_US') { ?>
-    <script type="text/javascript" src="ajax.php/i18n/<?php
+    <script type="text/javascript" src="<?php echo ROOT_PATH; ?>ajax.php/i18n/<?php
         echo $lang; ?>/js"></script>
 <?php } ?>
 <script type="text/javascript">