clang: suppress a warning

Use uint32_t for length.

    lib/token.c:286:29: warning: passing 'int32_t *'
          (aka 'int *') to parameter of type 'unsigned int *' converts between
          pointers to integer types with different sign [-Wpointer-sign]
                                &(tokenizer->len));
                                ^~~~~~~~~~~~~~~~~
clang: suppress a warning

Add a missing cast.

    lib/token.c:435:54: warning: passing
          'const unsigned char *' to parameter of type 'const char *' converts
          between pointers to integer types with different sign [-Wpointer-sign]
      grn_tokenizer_token_push(ctx, &(tokenizer->token), p, r - p, status);
                                                         ^
    include/groonga/tokenizer.h:183:61: note:
          passing argument to parameter 'str_ptr' here
      ...const char *str_ptr, unsigned int str_length,
                     ^