thermal/drivers/clock_cooling: Include export.h

It is preferrable 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/25f16415ab7b7587a052f1bce4133da318d58192.1589199124.git.amit.kucheria@linaro.org

drivers/thermal/clock_cooling.c

@@ -16,6 +16,7 @@
 #include <linux/cpufreq.h>
 #include <linux/device.h>
 #include <linux/err.h>
+#include <linux/export.h>
 #include <linux/idr.h>
 #include <linux/mutex.h>
 #include <linux/pm_opp.h>