Pass delta to MLT blocks' theme function.

With hook_apachesolr_modify_query, I can modify the morelikethis query to add more fields to the result output, but I cannot override the theme function to use these fields on some MLT blocks but not others.

This patch passes $delta from hook_block to the theme function.
(It would be nice to figure out how to know which MLT block's query is being altered, too, but I couldn't find a way to do this.)