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>

MAINTAINERS

@@ -4707,6 +4707,7 @@ L:	linux-iio@vger.kernel.org
 S:	Maintained
 F:	drivers/iio/
 F:	drivers/staging/iio/
+F:	include/linux/iio/
 
 IKANOS/ADI EAGLE ADSL USB DRIVER
 M:	Matthieu Castet <castet.matthieu@free.fr>