osdep.h: For Haiku, define SIGIO as equivalent to SIGPOLL

osdep.h: For Haiku, define SIGIO as equivalent to SIGPOLL

Haiku doesn't provide SIGIO; fix this up in osdep.h by defining it as
equal to SIGPOLL.

Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200703145614.16684-6-peter.maydell@linaro.org
[PMM: Expanded commit message]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>