Add back RegistrantViewsData file

modules/recurring_events_registration/src/Entity/RegistrantViewsData.php

+<?php
+
+namespace Drupal\recurring_events_registration\Entity;
+
+use Drupal\views\EntityViewsData;
+
+/**
+ * Provides Views data for Registrant entities.
+ */
+class RegistrantViewsData extends EntityViewsData {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getViewsData() {
+    $data = parent::getViewsData();
+
+    // Additional information for Views integration, such as table joins, can be
+    // put here.
+    return $data;
+  }
+
+}