Staging: iio: Added define guards where needed

The following files were added define guards to prevent multiple
inclusion.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --cc drivers/staging/iio/Documentation/iio_utils.h
Simple merge

diff --cc drivers/staging/iio/accel/sca3000.h
Simple merge

diff --cc drivers/staging/iio/frequency/dds.h
Simple merge

diff --cc drivers/staging/iio/meter/meter.h
Simple merge

diff --cc drivers/staging/iio/resolver/ad2s1210.h
Simple merge