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,
                     ^

lib/token.c

@@ -162,7 +162,7 @@ delimited_next(grn_ctx *ctx, int nargs, grn_obj **args, grn_user_data *user_data
     tokenizer->next =
       grn_tokenizer_tokenized_delimiter_next(ctx,
                                              &(tokenizer->token),
-                                             tokenizer->next,
+                                             (const char *)tokenizer->next,
                                              rest_length,
                                              tokenizer->query->encoding);
   } else {