include/hw/or-irq.h: Add missing include guard

The or-irq.h header file is missing the customary guard against
multiple inclusion, which means compilation fails if it gets
included twice. Fix the omission.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180220180325.29818-11-peter.maydell@linaro.org