iio: Export I2C module alias information

The I2C drivers have an i2c_device_id array but that information isn't
exported to the modules using the MODULE_DEVICE_TABLE() macro. So the
modules autoloading won't work if the I2C device is registered using
OF or legacy board files due missing alias information in the modules.

The issue was found using Kieran Bingham's coccinelle semantic patch:
https://lkml.org/lkml/2016/5/10/520

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/humidity/htu21.c

@@ -236,6 +236,7 @@ static const struct i2c_device_id htu21_id[] = {
 	{"ms8607-humidity", MS8607},
 	{}
 };
+MODULE_DEVICE_TABLE(i2c, htu21_id);
 
 static struct i2c_driver htu21_driver = {
 	.probe = htu21_probe,