iio: add IIO_TRIGGER dependency to STK8BA50

Ran into this on UML:

drivers/iio/accel/stk8ba50.c: In function ‘stk8ba50_data_rdy_trigger_set_state’:
drivers/iio/accel/stk8ba50.c:163:9: error: implicit declaration of function ‘iio_trigger_get_drvdata’ [-Werror=implicit-function-declaration]

iio_trigger_get_drvdata() is defined only when IIO_TRIGGER is selected.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>