rss_converter_twitter.com.xsl: restrict tweet selection some more

Only select elements which have the 'data-item-id' attribute, this way
we avoid picking up the image gallery at the top of hashtag pages which
does not have an RSS item structure.

JFTR the gallery is inside an element like this:

  <li class="AdaptiveStreamImageGallery AdaptiveSearchTimeline-separationModule js-stream-item"
      data-item-type="tweet">

with no 'data-item-id'.