iio: imu: st_lsm6dsx: st_lsm6dsx: Mark 'st_lsm6dsx_accel_ext_info' as __maybe_unused

It doesn't make sense to move it into '*core*' as it's co-located with
other, similar definitions which are used in multiple locations.

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_i2c.c:17:
 drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h:497:44: warning: ‘st_lsm6dsx_accel_ext_info’ defined but not used [-Wunused-const-variable=]
 497 | static const struct iio_chan_spec_ext_info st_lsm6dsx_accel_ext_info[] = {
 | ^~~~~~~~~~~~~~~~~~~~~~~~~
 In file included from drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_spi.c:17:
 drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h:497:44: warning: ‘st_lsm6dsx_accel_ext_info’ defined but not used [-Wunused-const-variable=]
 497 | static const struct iio_chan_spec_ext_info st_lsm6dsx_accel_ext_info[] = {
 | ^~~~~~~~~~~~~~~~~~~~~~~~~
 In file included from drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_i3c.c:16:
 drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h:497:44: warning: ‘st_lsm6dsx_accel_ext_info’ defined but not used [-Wunused-const-variable=]
 497 | static const struct iio_chan_spec_ext_info st_lsm6dsx_accel_ext_info[] = {
 | ^~~~~~~~~~~~~~~~~~~~~~~~~

Cc: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>