Initialize ATTRIBUTE_BLOCK variable only if not defined
Else reconfigure may show warnings like
Compiling Cookbooks...
/opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/deprecations.rb:4: warning: already initialized constant Class::ATTRIBUTE_BLOCKS
/opt/gitlab/embedded/cookbooks/cache/cookbooks/package/libraries/deprecations.rb:4: warning: previous definition of ATTRIBUTE_BLOCKS was here
Regression from !3565 (merged)