MAINTAINERS: Add IIO include files

Files under include/linux/iio were not reported as part
of the IIO subsystem.

Reported-by: Cristina Ciocan <cristina.ciocan@intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>