(AIX) (#11581) Fix undefined method "list_all" error on AIX user provider.

Without this patch, `puppet resource user` fails with the following
error:

`./lib/puppet/provider/aixobject.rb:291:in 'instances': undefined
method 'list_all' for Puppet::Type::User::ProviderAix:Class (NoMethodError)`

This patch fixes the problem by fixing the method definition which
makes the above command succeed and list all of the user resources.

Sprintly: fix ticket:203