clang: suppress a warning
clang: suppress a warning

Add a missing cast.

    lib/token.c:165:46: warning: passing
          'const unsigned char *' to parameter of type 'const char *' converts
          between pointers to integer types with different sign [-Wpointer-sign]
                                                 tokenizer->next,
                                                 ^~~~~~~~~~~~~~~
    include/groonga/tokenizer.h:195:82: note:
          passing argument to parameter 'str_ptr' here
      ...const char *str_ptr,
                     ^

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