Specialization of struct std::hash in different namespace

[ggahan]

Attempt at specialization of std::hash outside of proper namespace error encountered with examples.

Example being run: https://github.com/zeromq/cppzmq/blob/master/examples/pubsub_multithread_inproc.cpp

CENTOS 7
gcc 4.8.5

In file included from /home/grigori/Desktop/zeromq_test/main.cpp:7:0:
/home/grigori/Desktop/zeromq_test/zmq_addon.hpp:88:25: error: specialization of ‘template struct std::hash’ in different namespace [-fpermissive]
template <> struct std::hashzmq::poller_ref_t

Recommended fix:
enclose this template in namespace std as:

namespace std
{
  template <> struct hash<zmq::poller_ref_t>
  {
    size_t operator()(const zmq::poller_ref_t& ref) const ZMQ_NOTHROW
    {
      return ref.hash();
    }
  };
}