Add extern "C" around <net/if.h> on HPUX platform.

On hpux platform, functions(such as `if_nametoindex` and `if_indextoname`) in net/if.h doesn't declare with `extern "C"`. Fix this bug by add `extern "C"`. See http://www.boost.org/doc/libs/1_40_0/boost/asio/detail/socket_ops.hpp