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,
                     ^

lib/token.c

@@ -432,7 +432,11 @@ ngram_next(grn_ctx *ctx, int nargs, grn_obj **args, grn_user_data *user_data)
     tokenizer->skip = tokenizer->overlap ? 1 : len;
   }
   if (r == e) { status |= GRN_TOKENIZER_TOKEN_REACH_END; }
-  grn_tokenizer_token_push(ctx, &(tokenizer->token), p, r - p, status);
+  grn_tokenizer_token_push(ctx,
+                           &(tokenizer->token),
+                           (const char *)p,
+                           r - p,
+                           status);
   return NULL;
 }