Virtual default destructor to avoid compiler warning

The warning was

```
./rtbkit/common/testing/bid_request_synth.cc: In destructor ‘RTBKIT::Synth::NodeObject::~NodeObject()’:
./rtbkit/common/testing/bid_request_synth.cc:267:26: error: deleting object of abstract class type ‘RTBKIT::Synth::Node’ which has non-virtual destructor will cause undefined behaviour [-Werror=delete-non-virtual-dtor]
             delete field.second;
                          ^
cc1plus: all warnings being treated as errors
```