oslib-posix: fix compilation on OpenBSD

si_band is not found in OpenBSD.   It is marked as obsolescent in
POSIX, so we can delete it without any remorse.
Reported-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-id: 20170317152214.6148-1-pbonzini@redhat.com
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

util/oslib-posix.c

@@ -709,8 +709,6 @@ void sigaction_invoke(struct sigaction *action,
         si.si_pid = info->ssi_pid;
         si.si_status = info->ssi_status;
         si.si_uid = info->ssi_uid;
-    } else if (info->ssi_signo == SIGIO) {
-        si.si_band = info->ssi_band;
     }
     action->sa_sigaction(info->ssi_signo, &si, NULL);
 }