staging: iio: Documentation: Introduce iio_utils.c

This patch removes inline functions from iio_utils.h in order to clean the
code. iio_utils.c contains the implementation of the functions used by
iio_event_monitor.c, lsiio.c or generic_buffer.c and iio_utils.h contains
the declarations of these functions.

Since iio_utils.h is modified, generic_buffer.c and iio_event_monitor.c
must include stdlib.h.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>