split socket.hpp up in multiple headers to lower build time somewhat

src/natpmp.cpp

@@ -45,6 +45,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/io.hpp"
 #include "libtorrent/assert.hpp"
 #include "libtorrent/enum_net.hpp"
+#include "libtorrent/socket_io.hpp"
 
 using boost::bind;
 using namespace libtorrent;