iio/test-format: build kunit tests without structleak plugin

The structleak plugin causes the stack frame size to grow immensely when
used with KUnit:

../drivers/iio/test/iio-test-format.c: In function ‘iio_test_iio_format_value_fixedpoint’:
../drivers/iio/test/iio-test-format.c:98:1: warning: the frame size of 2336 bytes is larger than 2048 bytes [-Wframe-larger-than=]

Turn it off in this file.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>