iio: adc: move qcom-vadc-common.h to include dir

qcom-vadc-common module will be used by ADC thermal monitoring driver,
so move it to global include dir.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201204025509.1075506-6-dmitry.baryshkov@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>