iio: magnetometer: ak8975: update headers per IWYU principle

iio: magnetometer: ak8975: update headers per IWYU principle

Remove kernel.h proxy header and unused headers (slab.h, iio/sysfs.h,
iio/trigger.h). Add missing headers to ensure atomicity (array_size.h,
dev_printk.h, asm/byteorder.h, irqreturn.h, minmax.h, property.h,
types.h, wait.h).

Audited using the include-what-you-use tool.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Joshua Crofts <joshua.crofts1@gmail.com>
Reviewed-by: Maxwell Doose <m32285159@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>