Add plugins.js to preview_iframe.tpl.

This is a – possibly temporary - fix. serendipity_editor.js depends
on this file, so it needs to be included in the preview iframe as
well.

See http://board.s9y.org/viewtopic.php?p=10440336 (German only) for
further info on the issue this is related to.

templates/2k11/preview_iframe.tpl

@@ -27,6 +27,7 @@
 {/if}
     <script src="{serendipity_getFile file="js/modernizr-2.7.1.min.js"}"></script>
     {serendipity_hookPlugin hook="backend_header" hookAll="true"}
+    <script src="{serendipity_getFile file='admin/js/plugins.js'}"></script>
     <script src="{serendipity_getFile file='admin/serendipity_editor.js'}"></script>
 <script>
     window.onload = function() {ldelim}