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>
6 jobs for yk/fix-boost-distance in 203 minutes and 28 seconds
latest
Status Job ID Name Coverage
  Test
failed #58312
archlinux:clang:debug

00:06:52

failed #36539
archlinux:gcc:speed

00:08:55

passed #36542
debian:clang:speed

01:29:41

canceled #58313
debian:gcc:debug

01:46:53

failed #36540
archlinux:clang:debug

00:09:56

passed #36541
debian:gcc:debug

02:03:57

 
Name Stage Failure
failed
archlinux:clang:debug Test
make[1]: *** [Makefile:3401: lib/vcsn/rat/libvcsn_la-parse.lo] Error 1
make[1]: Leaving directory '/builds/vcsn/vcsn/build'
make: *** [Makefile:2360: all] Error 2
Uploading artifacts...
build/*.log: found 1 matching files
WARNING: build/*/*.log: no matching files
WARNING: build/*/*/*.log: no matching files
Uploading artifacts to coordinator... ok
id=58312 responseStatus=201 Created token=6Brcefte
ERROR: Job failed: exit code 1
failed
archlinux:gcc:speed Test
make[1]: *** [Makefile:3401: lib/vcsn/rat/libvcsn_la-parse.lo] Error 1
make[1]: Leaving directory '/builds/vcsn/vcsn/build'
make: *** [Makefile:2360: all] Error 2
Uploading artifacts...
build/*.log: found 1 matching files
WARNING: build/*/*.log: no matching files
WARNING: build/*/*/*.log: no matching files
Uploading artifacts to coordinator... ok
id=36539 responseStatus=201 Created token=J7Rwo2Eb
ERROR: Job failed: exit code 1