thermal: st: allow compile test

Adding COMPILE_TEST flag to st driver to facilitate
maintenance.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Nicolas Boichat <drinkcat@chromium.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Wolfram Sang <wsa@the-dreams.de>
Cc: Lee Jones <lee.jones@linaro.org>
Cc: linux-pm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>