iio:triggers Protect functions in triggers.h from use when not compiled

Also include a couple of forward defs of struct iio_trigger and struct
iio_trigger_ops to avoid doing this in each driver.

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