iio: event_monitor: Flush output on event

By flushing the output, iio_event_monitor can be more useful to programs
chained along with it.

  iio_event_monitor stk3310 | awk '/rising/{system("my_unlockscreen.sh")} /falling/{system("my_lockscreen.sh")}'

Without this flush, the above example would buffer a number of events,
then after a while run the lock/unlock scripts several times.

Signed-off-by: Zach DeCook <zachdecook@librem.one>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>