iio: merge hid-sensor-attributes.h into hid-sensor-hub.h

The stuff in hid-sensor-attributes.h is needed by every piece which
uses hid-sensor-hub and merging it into hid-sensor-hub.h makes it accessible
from outside the iio subdirectory.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>