dnsdist: Fix build error when only protobuf is enabled

dnsdist: Fix build error when only protobuf is enabled

The build fails with:
```
../dnsdist-protobuf.cc: In member function ‘void DNSDistProtoBufMessage::serialize(std::string&) const’:
../dnsdist-protobuf.cc:205:7: error: ‘vinfolog’ was not declared in this scope
  205 |       vinfolog("Error while parsing the RRs from a response packet to add them to the protobuf message: %s", exp.what());
      |       ^~~~~~~~
```
because of a missing header.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>