iio: test: fix missing MODULE_LICENSE for IIO_RESCALE=m

When IIO_RESCALE_KUNIT_TEST=y and IIO_RESCALE=m,
drivers/iio/afe/iio-rescale.o is built twice causing the
MODULE_LICENSE() to be lost, as shown by:

  ERROR: modpost: missing MODULE_LICENSE() in drivers/iio/afe/iio-rescale.o

Rework the build configuration to have the dependency specified in the
Kconfig.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: 8e74a48d

 ("iio: test: add basic tests for the iio-rescale driver")
Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20220601142138.3331278-1-liambeguin@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>