Fix two more warnings

In file included from dnswriter.cc:1:
./dnswriter.hh:119:12: warning: private field 'd_qtype' is not used [-Wunused-private-field]
  uint16_t d_qtype, d_qclass;
           ^
./dnswriter.hh:119:21: warning: private field 'd_qclass' is not used [-Wunused-private-field]
  uint16_t d_qtype, d_qclass;
                    ^