iio: Kconfig: at91_adc: add COMPILE_TEST dependency to driver

Since changes can come from all sort of places, it may make sense to have
this symbol as a dependency to make sure that the 'make allmodconfig' &&
'make allyesconfig' build rules cover this driver as well for a
compile-build/test.

It seemed useful [recently] when trying to apply a change for this.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>