erratum for GotW #62 (pimpl-idiom and std::auto_ptr)

Sutter's page for the guru question GotW #62 reads:

"... this is not so good if Y is a Pimpl, for example, and the whole point is to hide Y's definition from clients of X2".


If you use a forward-decl. for Y you don't need to show around the def. for Y

see also GotW #59