clang: suppress a warning

Add a missing cast.

    lib/token.c:494:15: warning: assigning to
          'const unsigned char *' from 'const char *' converts between pointers to
          integer types with different sign [-Wpointer-sign]
      token->orig = str;
                  ^ ~~~
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_);
                      ^ ~~~~~~~~~~~~~~~~~~~~~