minor selector change to avoid iframe security violation in UI.init() for customized/forked skins

If someone updates the header or footer templates to insert an ad banner or somesuch as an iframe and uses the sandbox attribute to prevent that iframe from breaking into the roundcube content, the browser (chrome v33, at least) generates a security exception that prevents UI.init() from finishing.

This minor change to the selector makes it skip any iframes that might cause this problem.