Full pager plugin: reference the 'pager' theme in the annotations.

Coming from #2030293: View preview is broken in UI if a pager has to be displayed, comment #36.

+++ b/core/modules/views/lib/Drupal/views/Plugin/views/pager/Full.phpundefined
@@ -90,13 +89,13 @@ function render($input) {
+      '#theme' => $this->view->buildThemeFunctions('pager'),

As a follow up I would suggest to use $this->themeFunctions which would require a "theme = 'pager' and register_theme = FALSE" in the annotation.