iio: imu: st_lsm6dsx: fix build when CONFIG_IIO_TRIGGERED_BUFFER=m

The following kernel linkage error:

st_lsm6dsx_core.o: in function `st_lsm6dsx_sw_buffers_setup':
st_lsm6dsx_core.c:2578: undefined reference to `devm_iio_triggered_buffer_setup_ext'

is caused by the fact that the object owning devm_iio_triggered_buffer_setup_ext()
(drivers/iio/buffer/industrialio-triggered-buffer.o) is allowed to be
built as module when its user (drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c)
is built-in.

The st_lsm6dsx driver already has a "select IIO_BUFFER", so add another
select for IIO_TRIGGERED_BUFFER, to make that option follow what is set
for the st_lsm6dsx driver. This is similar to what other iio drivers do.

Fixes: 2cfb2180c3e8 ("iio: imu: st_lsm6dsx: introduce sw trigger support")
Signed-off-by: Vladimir Oltean <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Cc: <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/imu/st_lsm6dsx/Kconfig

···
 	tristate "ST_LSM6DSx driver for STM 6-axis IMU MEMS sensors"
 	depends on (I2C || SPI || I3C)
 	select IIO_BUFFER
+	select IIO_TRIGGERED_BUFFER
 	select IIO_KFIFO_BUF
 	select IIO_ST_LSM6DSX_I2C if (I2C)
 	select IIO_ST_LSM6DSX_SPI if (SPI_MASTER)