MAINTAINERS: add IIO include files

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

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Reported-by: Cristina Ciocan <cristina.ciocan@intel.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -4716,6 +4716,7 @@ 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>