asio: Add missing LIBPORT_API causing weird dynamic_cast failures.
asio: Add missing LIBPORT_API causing weird dynamic_cast failures.

The failure only happens when inculding and using asio.hh from multiple
compilation units.

	* include/libport/asio.hxx: Declare SocketImpl and
	SocketImplBase as LIBPORT_API.

aldebaran/libport,aldebaran/libport,aldebaran/libport,aldebaran/libport,aldebaran/libport