(#16347) Qualify references to windows-pr gem

Previously, running gem install puppet on ruby 1.9.3, would fail when
the Puppet::Util::Windows::Security and Puppet::Util::Windows::User
modules both included the Windows::Security module from the windows-pr
gem.

The failure occurs when rdoc attempts to generate documentation for
Puppet::Util::Windows::User, and gets into an infinite loop, eventually
causing a stack overflow.

This commit fully qualifies to windows-specific gems so that this
doesn't occur in the future.

Paired-with: Andy Parker <andy@puppetlabs.com>