staging: iio: Add #include guards to header files

This patch adds #include guards in order to make the header files idempotent.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>