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 77712e5f

: ("Staging: iio: Staticise non-exported functions")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>