Add index.php file to googlemerchant XML module with redirect headers

This commit introduces a new index.php file in the googlemerchant XML module. The file contains headers to prevent caching and redirects users to the parent directory. This is part of the module's structure to ensure proper access control and user experience.

modules/x13googlemerchant/views/templates/admin/tab.tpl

@@ -0,0 +1,5 @@
+{if version_compare($smarty.const._PS_VERSION_, '1.6.0.0', '<')}
+    {include file="./tab_15.tpl"}
+{else}
+    {include file="./tab_16.tpl"}
+{/if}