MAINTAINERS: Move Hartmut Knaack to Credits

MAINTAINERS: Move Hartmut Knaack to Credits

Hartmut Knaack was an active reviewer and contributor to the IIO
subsystem and drivers.  However his last message on LKML is from
October 2015.

In thanks for Hartmut's effort, move him name to the Credits.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio <linux-iio@vger.kernel.org>
Link: https://lore.kernel.org/r/20200903181926.5606-2-krzk@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>