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 <[e-mail]>
Cc: Liam Beguin <[e-mail]>
Reviewed-by: Liam Beguin <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/iio/test/iio-test-rescale.c

···
 	const char *expected_off;
 };
 
-const struct rescale_tc_data scale_cases[] = {
+static const struct rescale_tc_data scale_cases[] = {
 	/*
 	 * Typical use cases
 	 */
···
 	},
 };
 
-const struct rescale_tc_data offset_cases[] = {
+static const struct rescale_tc_data offset_cases[] = {
 	/*
 	 * Typical use cases
 	 */