clang: suppress a warning

Use "uint8_t *" for key buffer.

    lib/pat.c:2904:36: warning: passing
          'uint8_t [4096]' to parameter of type 'char *' converts between pointers
          to integer types with different sign [-Wpointer-sign]
      byte_len = rk_conv(key, key_len, keybuf, GRN_TABLE_MAX_KEY_SIZE, &state);
                                       ^~~~~~
    lib/pat.c:2780:50: note: passing argument to
          parameter 'buf' here
      ...*str, uint32_t str_len, char *buf, uint32_t buf_size, uint8_t *statep)
                                       ^