thermal: Change IS_ENABLED to IFDEF in the header file

The thermal framework can not be compiled as a module. The IS_ENABLED
macro is useless here and can be replaced by an ifdef.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20200402142747.8307-7-daniel.lezcano@linaro.org