sandbox: adc: Add missing header file

Commit: sandbox: add ADC driver

adds the driver without its main header file.
It causes build brake for sandbox_defonfig.

This commit adds a missing header:
- include/sandbox-adc.h

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>