Append rather than override classes in theme_preprocess_webform_time()

Same as #2030869: Theme cannot add classes via preprocess webform date

Theme code

/**
 * Implements theme_preprocess_webform_time().
 */
function foo_preprocess_webform_time(&$variables) {
  // #2030885: Theme cannot add classes via theme_preprocess_webform_time().
  $variables['element']['hour']['#attributes']['class'][] = 'foo-inline';
  $variables['element']['minute']['#attributes']['class'][] = 'foo-inline';
  $variables['element']['ampm']['#attributes']['class'][] = 'foo-inline';

Broken code:

/**
 * Theme a webform time element.
 */
function theme_webform_time($variables) {
  $element = $variables['element'];

  $element['hour']['#attributes']['class'] = array('hour');
  $element['minute']['#attributes']['class'] = array('minute');

Bugfix:

/**
 * Theme a webform time element.
 */
function theme_webform_time($variables) {
  $element = $variables['element'];

  $element['hour']['#attributes']['class'][] = 'hour';
  $element['minute']['#attributes']['class'][] = 'minute';