iio: trigger: make stub functions static inline

Make sure that the trigger function stubs are all static inline.
Otherwise we might see compiler warnings about declared but unused
functions.

Fixes 77712e5fbe2e4: ("Staging: iio: Staticise non-exported functions")
Signed-off-by: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Alexandru Ardelean <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/iio_core_trigger.h

···
  * iio_device_register_trigger_consumer() - set up an iio_dev to use triggers
  * @indio_dev: iio_dev associated with the device that will consume the trigger
  **/
-static int iio_device_register_trigger_consumer(struct iio_dev *indio_dev)
+static inline int iio_device_register_trigger_consumer(struct iio_dev *indio_dev)
 {
 	return 0;
 }
···
  * iio_device_unregister_trigger_consumer() - reverse the registration process
  * @indio_dev: iio_dev associated with the device that consumed the trigger
  **/
-static void iio_device_unregister_trigger_consumer(struct iio_dev *indio_dev)
+static inline void iio_device_unregister_trigger_consumer(struct iio_dev *indio_dev)
 {
 }