optimized header dependencies to lower build time

include/libtorrent/udp_socket.hpp

@@ -34,6 +34,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #define TORRENT_UDP_SOCKET_HPP_INCLUDED
 
 #include "libtorrent/socket.hpp"
+#include "libtorrent/io_service.hpp"
 #include "libtorrent/error_code.hpp"
 #include "libtorrent/session_settings.hpp"
 #include "libtorrent/buffer.hpp"
@@ -41,7 +42,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/deadline_timer.hpp"
 
 #include <vector>
-#include <boost/function.hpp>
+#include <boost/function/function4.hpp>
 
 namespace libtorrent
 {