Add frozen string literal comment to files.

Add frozen string literal comment to files.

diff --git a/plugin.rb b/plugin.rb
index 7055bdb..156bd09 100644
--- a/plugin.rb
+++ b/plugin.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # name: discourse-sitemap
 # about: Generate XML sitemap for your Discourse forum.
 # version: 1.2

GitHub sha: ca1a7964