iio:adc:envelope-detector: Switch from of headers to mod_devicetable.h

There is nothing directly using of specific interfaces in this driver,
so lets not include the headers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>