slirp: Enable fork_exec support on Windows

g_spawn_async_with_fds is portable on Windows, so we can now enable
fork_exec support there.

Thanks Daniel P. Berrangé for the notice!

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>