rat: printer: fix build issue with Boost >= 1.69

Boost 1.69 introduced an ADL barrier on boost::distance(), so the call
that depended on it failed to build. Simply use qualified name lookup
instead.

See also https://github.com/boostorg/range/pull/75.

* vcsn/core/rat/printer.hh: Fix.

Reported-by: Baptiste Esteban <baptiste.esteban@epita.fr>