MDL-56888 workshop: Fix Leap2A export of the workshop submission

The whole story described in the tracker. Shortly, Mahara seems to have
a bug (or undocumented feature) that it expects the Leap2A content
wrapped in a single container.

mod/workshop/classes/portfolio_caller.php

@@ -315,7 +315,7 @@ class mod_workshop_portfolio_caller extends portfolio_module_caller_base {
             $output .= $content;
         }
 
-        return $output;
+        return html_writer::div($output);
     }
 
     /**