tools: iio: add .gitignore

The generated files must not be tracked by git. Add a local .gitignore.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

tools/iio/.gitignore

+iio_event_monitor
+iio_generic_buffer
+lsiio
+include/