staging: iio: add IIO_ALTVOLTAGE to iio_event_monitor example

IIO_ALTVOLTAGE is used by frequency/adf4350 and frequency/ad9523

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>