iio: imu: st_lsm6dsx: st_lsm6dsx: Reorder 'inline' declaration

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

 drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h:487:1: warning: ‘inline’ is not at beginning of declaration [-Wold-style-declaration]
 drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h:497:44: warning: ‘st_lsm6dsx_accel_ext_info’ defined but not used [-Wunused-const-variable=]

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>