iio: adc: ad_sigma_delta: audit included headers

iio: adc: ad_sigma_delta: audit included headers

Drop linux/iio/sysfs.h since it is unused and replace linux/kernel.h
with more explicit headers. There are a couple of other headers added
weren't covered by kernel.h, like linux/gpio/consumer.h that are added
since the module makes use of those APIs as well.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250701-iio-adc-ad7173-add-spi-offload-support-v3-6-42abb83e3dac@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>