Fix build on OpenBSD

[hboetes]

network.cc: In member function 'std::string Network::Connection::recv_one(int, bool)':
network.cc:419: error: aggregate 'iovec msg_iovec' has incomplete type and cannot be defined
*** Error code 1

This issue is fixed by this patch:

--- src/network/network.cc.orig Fri Oct 19 21:45:18 2012
+++ src/network/network.cc      Fri Feb 15 22:19:12 2013
@@ -34,6 +34,7 @@

 #include <sys/types.h>
 #include <sys/socket.h>
+#include <sys/uio.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
 #include <assert.h>