thermal: st: fix Makefile typo

When STM32_THERMAL is enabled, this overrides all previously
enabled files in the same directory, as seen from this link failure:

ERROR: "st_thermal_pm_ops" [drivers/thermal/st/st_thermal_syscfg.ko] undefined!
ERROR: "st_thermal_register" [drivers/thermal/st/st_thermal_syscfg.ko] undefined!
ERROR: "st_thermal_unregister" [drivers/thermal/st/st_thermal_syscfg.ko] undefined!

The correct syntax in Makefile requires using += instead of :=.

Fixes: 1d693155 ("thermal: add stm32 thermal driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>