iio: hid-sensors: respect CONFIG_IIO_TRIGGER

Not much to say, without that change, hid-sensor-trigger will be
always compiled if HID_SENSOR_IIO_COMMON is selected which fails if
CONFIG_IIO_TRIGGER is not set because CONFIG_IIO_CONSUMERS_PER_TRIGGER
will not be defined.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>