Add default constructor to xrt_graph.h

[dsp-avt]

Please add an empty default constructor to the xrt::graph class. This will make it easier to use an xrt::graph object within a user defined class.

Example:

class graph {
public:
  graph() = default;
  graph(const xrt::device& device, const xrt::uuid& xclbin_id, const std::string& name, access_mode am = access_mode::primary);
  ...
};