Style: Fix -Wmissing-field-initializers in socket_poller.cpp

This commit addresses the following warnings reported on gcc 5.2.1. In
the future, this will help reduce the "noise" and help catch warnings
revealing a serious problem.

It was originally introduce in the refactoring associated with
zeromq/libzmq@da2bc60 (Removing zmq_pollfd as it is replaced by zmq_poller).

8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---
/path/to/libzmq/src/socket_poller.cpp: In member function ‘int zmq::socket_poller_t::add(zmq::socket_base_t*, void*, short int)’:
/path/to/libzmq/src/socket_poller.cpp:92:51: warning: missing initializer for member ‘zmq::socket_poller_t::item_t::pollfd_index’ [-Wmissing-field-initializers]
     item_t item = {socket_, 0, user_data_, events_};
                                                   ^
/path/to/libzmq/src/socket_poller.cpp: In member function ‘int zmq::socket_poller_t::add_fd(zmq::fd_t, void*, short int)’:
/path/to/libzmq/src/socket_poller.cpp:108:50: warning: missing initializer for member ‘zmq::socket_poller_t::item_t::pollfd_index’ [-Wmissing-field-initializers]
     item_t item = {NULL, fd_, user_data_, events_};
                                                  ^
8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---

src/socket_poller.cpp

@@ -89,7 +89,11 @@ int zmq::socket_poller_t::add (socket_base_t *socket_, void* user_data_, short e
            return -1;
     }
 
-    item_t item = {socket_, 0, user_data_, events_};
+    item_t item = {socket_, 0, user_data_, events_
+#if defined ZMQ_POLL_BASED_ON_POLL
+                   ,-1
+#endif
+    };
     items.push_back (item);
     need_rebuild = true;
 
@@ -105,7 +109,11 @@ int zmq::socket_poller_t::add_fd (fd_t fd_, void *user_data_, short events_)
         }
     }
 
-    item_t item = {NULL, fd_, user_data_, events_};
+    item_t item = {NULL, fd_, user_data_, events_
+#if defined ZMQ_POLL_BASED_ON_POLL
+                   ,-1
+#endif
+                   };
     items.push_back (item);
     need_rebuild = true;