iio:st_sensors fix build when !CONFIG_IIO_TRIGGER

Partly a case of removing unused headers and partly a case
of ifdefing out the iio_trigger_ops structures.  This has
come about because of an 'unusual' separation of code in this
driver.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Denis Ciocca <denis.ciocca@st.com>