fix:iio:common:st_sensors:st_sensors_trigger:mark symbols static where possible

fix:iio:common:st_sensors:st_sensors_trigger:mark symbols static where possible

We get 2 warnings when biuld kernel with W=1:
drivers/iio/common/st_sensors/st_sensors_trigger.c:69:13: warning: no previous prototype
for 'st_sensors_irq_handler' [-Wmissing-prototypes]
drivers/iio/common/st_sensors/st_sensors_trigger.c:85:13: warning: no previous prototype
for 'st_sensors_irq_thread' [-Wmissing-prototypes]

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.
so this patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>