iio: inkern: Fix headers inclusion order

Fix the mutex.h inclusion order as it seems to be the only one
misplaces.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/20230623085830.749991-7-herve.codina@bootlin.com


Signed-off-by: Mark Brown <broonie@kernel.org>