Add frozen_string_literal to search_labels_spec.rb according to GitLab bot

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

spec/features/projects/labels/search_labels_spec.rb

+# frozen_string_literal: true
+
 require 'spec_helper'
 
 describe 'Search for labels', :js do