iio: gts-helper: export iio_gts_get_total_gain()

iio: gts-helper: export iio_gts_get_total_gain()

Export this function in preparation for the fix in veml6030.c, where the
total gain can be used to ease the calculation of the processed value of
the IIO_LIGHT channel compared to acquiring the scale in NANO.

Suggested-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://patch.msgid.link/20250127-veml6030-scale-v3-1-4f32ba03df94@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>