clang: suppress a warning

Use signed "char *" for string.

    lib/normalizer.c:660:51: warning: passing
          'const unsigned char *' to parameter of type 'const char *' converts
          between pointers to integer types with different sign [-Wpointer-sign]
            grn_tokenizer_is_tokenized_delimiter(ctx, s, ls, GRN_ENC_UTF8)) {
                                                      ^
    include/groonga/tokenizer.h:59:59: note: passing
          argument to parameter 'str_ptr' here
                                                  const char *str_ptr,
                                                              ^