iio: test: Mark file local structure arrays static.

Warning cleanup:
  drivers/iio/test/iio-test-rescale.c:32:30: warning: symbol 'scale_cases' was not declared. Should it be static?
  drivers/iio/test/iio-test-rescale.c:480:30: warning: symbol 'offset_cases' was not declared. Should it be static?

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220807145457.646062-2-jic23@kernel.org