clang: suppress a warning
clang: suppress a warning

Add a missing cast.

    lib/token.c:552:19: warning: assigning to
          'const unsigned char *' from 'char *' converts between pointers to integer
          types with different sign [-Wpointer-sign]
          token->curr = GRN_TEXT_VALUE(curr_);
                      ^ ~~~~~~~~~~~~~~~~~~~~~

cosmo0920/groonga,cosmo0920/groonga,naoa/groonga,redfigure/groonga,redfigure/groonga,naoa/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,komainu8/groonga,cosmo0920/groonga,komainu8/groonga,hiroyuki-sato/groonga,komainu8/groonga,komainu8/groonga,groonga/groonga,redfigure/groonga,hiroyuki-sato/groonga,hiroyuki-sato/groonga,myokoym/groonga,komainu8/groonga,kenhys/groonga,myokoym/groonga,cosmo0920/groonga,naoa/groonga,myokoym/groonga,komainu8/groonga,hiroyuki-sato/groonga,groonga/groonga,myokoym/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,myokoym/groonga,groonga/groonga,redfigure/groonga,redfigure/groonga,naoa/groonga,redfigure/groonga,myokoym/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,redfigure/groonga,naoa/groonga,groonga/groonga,groonga/groonga,kenhys/groonga,hiroyuki-sato/groonga,naoa/groonga,myokoym/groonga,naoa/groonga,redfigure/groonga,groonga/groonga,myokoym/groonga,cosmo0920/groonga,kenhys/groonga,naoa/groonga,groonga/groonga,komainu8/groonga,cosmo0920/groonga,komainu8/groonga,hiroyuki-sato/groonga,kenhys/groonga,kenhys/groonga,kenhys/groonga