thermal/drivers/thermal_helpers: Include export.h

It is preferable to include export.h when you are using EXPORT_SYMBOL
family of macros.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/fd3443f00dbba6ca90f35726c7451ae52145d2d4.1589199124.git.amit.kucheria@linaro.org