Extend C++11 guard to cover remaining use of std::vector data().

Now compiles without error with a pre-C++11 compiler (g++-3.3).

Fixes #53
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zmq.hpp

@@ -144,12 +144,12 @@ namespace zmq
     {
         return poll(items.data(), items.size(), timeout.count() );
     }
-    #endif
 
     inline int poll(std::vector<zmq_pollitem_t> const& items, long timeout_ = -1)
     {
         return poll(items.data(), items.size(), timeout_);
     }
+    #endif