'clear-block' CSS class on admin form should be 'clearfix'

Feeds admin UI uses floats, but these get covered up by any content below the admin UI, such as a block in the footer region.

The code tries to fix this, but uses the wrong CSS class:

  // Outer wrapper.
  $output = '<div class="feeds-settings clear-block">';

D7 changed this CSS class to 'clearfix'.