iio:accel:kxcjk-1013: Mark struct __maybe_unused to avoid warning.

This structure is only used in PM ops, so may not be used depending
on build configuration.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20211128172445.2616166-13-jic23@kernel.org