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>