thermal/drivers/thermal_hwmon: Include export.h

It is preferable 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/f542962494a8441fdc8e550a11d0e535b92362a0.1589199124.git.amit.kucheria@linaro.org

drivers/thermal/thermal_hwmon.c

@@ -11,6 +11,7 @@
  *  Copyright (C) 2013 Eduardo Valentin <eduardo.valentin@ti.com>
  */
 #include <linux/err.h>
+#include <linux/export.h>
 #include <linux/hwmon.h>
 #include <linux/slab.h>
 #include <linux/thermal.h>