make poller_t work with ancient gcc 4.8.1
make poller_t work with ancient gcc 4.8.1

For some reason that I didn't get to the root cause, gcc 4.8.1 (that I'm
stuck with) does not like the initializer for `unique_ptr` implemented
as a lambda:

```
third_party/zmqcpp/repo/zmq.hpp: In constructor 'zmq::poller_t<T>::poller_t() [with T = std::function<void(zmq::event_flags)>]':
third_party/zmqcpp/repo/zmq.hpp:1871:5: error: converting to 'std::unique_ptr<void, zmq::poller_t<std::function<void(zmq::event_flags)> >::destroy_poller_t>' from initializer list would use explicit constructor 'std::unique_ptr<_Ty, _Dx>::unique_ptr(std::unique_ptr<_Ty, _Dx>::pointer) [with _Ty = void; _Dx = zmq::poller_t<std::function<void(zmq::event_flags)> >::destroy_poller_t; std::unique_ptr<_Ty, _Dx>::pointer = void*]'
     poller_t() = default;
     ^
In file included from networking/ipc/ipc.cc:6:0:
third_party/zmqcpp/repo/zmq_addon.hpp: At global scope:
third_party/zmqcpp/repo/zmq_addon.hpp:447:40: note: synthesized method 'zmq::poller_t<T>::poller_t() [with T = std::function<void(zmq::event_flags)>]' first required here
     poller_t<handler_type> base_poller{};
                                        ^
In file included from ./networking/ipc/ipc.h:13:0,
                 from networking/ipc/ipc.cc:1:
third_party/zmqcpp/repo/zmq.hpp: In constructor 'zmq::poller_t<T>::poller_t() [with T = zmq::socket_t]':
third_party/zmqcpp/repo/zmq.hpp:1871:5: error: converting to 'std::unique_ptr<void, zmq::poller_t<zmq::socket_t>::destroy_poller_t>' from initializer list would use explicit constructor 'std::unique_ptr<_Ty, _Dx>::unique_ptr(std::unique_ptr<_Ty, _Dx>::pointer) [with _Ty = void; _Dx = zmq::poller_t<zmq::socket_t>::destroy_poller_t; std::unique_ptr<_Ty, _Dx>::pointer = void*]'
     poller_t() = default;
     ^
networking/ipc/ipc.cc: In member function 'void networking::ipc::Ipc::ThreadMain()':
networking/ipc/ipc.cc:313:36: note: synthesized method 'zmq::poller_t<T>::poller_t() [with T = zmq::socket_t]' first required here
   ::zmq::poller_t<::zmq::socket_t> poller;
                                    ^
```

This moves the initialization to constructor, and makes gcc happy.

zeromq/cppzmq,zeromq/cppzmq