osmo_io.h: add forward declaration of struct osmo_sockaddr
During amr-none-eabi build (EMBEDDED defined), including <osmocom/core/socket.h> includes a mostly empty file, since that file is guarded by "#if (!EMBEDDED)" because sys/socket.h is not available there. As a result, if somebody includes osmo_io.h when EMBEDDED is set, there will be no declaration of struct osmo_sockaddr used in declared APIs. Add a forward declaration to fix it.
