ath10k: Fix build with CONFIG_THERMAL=m

After commit cf4f2193 ("kbuild: Allow to specify composite modules
with modname-m"), thermal.c gets included in the driver and the build
fails with

drivers/net/wireless/ath/ath10k/thermal.c:119:6: error: redefinition of ‘ath10k_thermal_event_temperature’
drivers/net/wireless/ath/ath10k/thermal.h:54:20: note: previous definition of ‘ath10k_thermal_event_temperature’ was here
drivers/net/wireless/ath/ath10k/thermal.c:136:6: error: redefinition of ‘ath10k_thermal_set_throttling’
drivers/net/wireless/ath/ath10k/thermal.h:59:20: note: previous definition of ‘ath10k_thermal_set_throttling’ was here
drivers/net/wireless/ath/ath10k/thermal.c:162:5: error: redefinition of ‘ath10k_thermal_register’
drivers/net/wireless/ath/ath10k/thermal.h:45:19: note: previous definition of ‘ath10k_thermal_register’ was here
drivers/net/wireless/ath/ath10k/thermal.c:216:6: error: redefinition of ‘ath10k_thermal_unregister’
drivers/net/wireless/ath/ath10k/thermal.h:50:20: note: previous definition of ‘ath10k_thermal_unregister’ was here

Change the #ifdef to reflect the new kbuild behavior.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Michal Marek <mmarek@suse.com>

drivers/net/wireless/ath/ath10k/thermal.h

@@ -36,7 +36,7 @@ struct ath10k_thermal {
 	int temperature;
 };
 
-#ifdef CONFIG_THERMAL
+#if IS_REACHABLE(CONFIG_THERMAL)
 int ath10k_thermal_register(struct ath10k *ar);
 void ath10k_thermal_unregister(struct ath10k *ar);
 void ath10k_thermal_event_temperature(struct ath10k *ar, int temperature);