ipa: Add forward declaration of struct osmo_fd

A pointer to struct osmo_fd is used in a few prototypes declared in
gsm/ipa.h, but is neither declared explicitly nor is such a
declaration reachable via the given include directives.

This patch adds a forward declaration of this type to ensure proper
compilation.

Sponsored-by: On-Waves ehf
ipa: Return -errno instead of -1 in ipa_send

Currently, the ipa_send function returns -1 in one execution branch
to indicate an error and -EIO in another. This is not consistent and
can lead to a misinterpretation of the error code, since -1 is -EPERM
and in general, EPERM is not returned by write(2).

This patch changes the return code to -errno instead of -1 for the
case that write(2) fails for same reason. So -rc is always a sensible
error value if there is a failure.

Sponsored-by: On-Waves ehf