util/compatfd.c includes <sys/syscall.h> so that the CONFIG_SIGNALFD
code can use SYS_signalfd. Guard the #include with CONFIG_SIGNALFD
to avoid portability issues on hosts like Haiku which do not
provide that header file.

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: 20200703145614.16684-8-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>