iio: Fix build error seen if IIO_TRIGGER is defined but IIO_BUFFER is not

If CONFIG_IIO_TRIGGER is defined but CONFIG_IIO_BUFFER is not, the following
build error is seen.

drivers/iio/common/st_sensors/st_sensors_trigger.c:21:5: error:
redefinition of ‘st_sensors_allocate_trigger’
In file included from
drivers/iio/common/st_sensors/st_sensors_trigger.c:18:0:
include/linux/iio/common/st_sensors.h:239:19: note: previous
definition of ‘st_sensors_allocate_trigger’ was here
drivers/iio/common/st_sensors/st_sensors_trigger.c:65:6: error:
redefinition of ‘st_sensors_deallocate_trigger’
In file included from
drivers/iio/common/st_sensors/st_sensors_trigger.c:18:0:
include/linux/iio/common/st_sensors.h:244:20: note: previous
definition of ‘st_sensors_deallocate_trigger’ was here

This occurs because st_sensors_deallocate_trigger is built if CONFIG_IIO_TRIGGER
is defined, but the dummy function is compiled if CONFIG_IIO_BUFFER is defined.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>