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));
                                ^~~~~~~~~~~~~~~~~

lib/token.c

@@ -238,7 +238,7 @@ typedef struct {
   const unsigned char *next;
   const unsigned char *end;
   const uint_least8_t *ctypes;
-  int32_t len;
+  uint32_t len;
   uint32_t tail;
 } grn_ngram_tokenizer;