RuboCop: exclude Guardfile from list of files

This version of RuboCop failed to execute when reading that file, with
this error:

An error occurred while Layout/BlockAlignment cop was inspecting
/home/runner/work/annotate_models/annotate_models/Guardfile:9:0.

So, ignoring that, for now.

.rubocop.yml

@@ -11,6 +11,7 @@ AllCops:
     - 'spec/fixtures/**/*'
     - 'tmp/**/*'
     - 'spec/integration/**/*'
+    - 'Guardfile'
   NewCops: enable
 
 Metrics/BlockLength: