Changes

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.

Furthermore, the EMBEDDED check in osmocom/core/socket.h is actually
broken because it's in a header file which cannot include "config.h", so
also avoid including osocom/core/socket.h altogether in order to avoid
it in turn including arpa/inet.h which is not available in the
arm-none-eabi toolchain.

Change-Id: Id7db4b081752a1034b95a92b4850484a26ec11c6