hwmon: (lm75.h) Update header inclusion

File lm75.h used to include <linux/hwmon.h> for SENSORS_LIMIT() but
this function is gone by now. Instead we call clamp_val() so we should
include <linux/kernel.h>, where this function is declared.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>