Fixes #7068/bz1129775 - Fix repo sync checksum error

When one synced a repo priori to this commit, we got an error that
looked like

Error
undefined method `hidden' for #<Class:0x0000000d6beab0> (NoMethodError)
gems/ruby-2.0.0-p481/gems/activerecord-3.2.18/lib/active_record/dynamic_matchers.rb:55:in
`method_missing'
/katello/app/lib/actions/katello/repository/correct_checksum.rb:23:in
`finalize'

Basically after the commit for #6870/BZ1075523, all instance of the
User.hidden was replaced with User.anonymous_admin in katello code.
Sounds like there was a missed spot in this file
/katello/app/lib/actions/katello/repository/correct_checksum.rb.

this commit fixes this.
"
Merge pull request #4576 from parthaa/hidden-fix

Fixes #7068/bz1129775 - Fix repo sync checksum error