hwmon: (it87) Add missing sysfs attribute group terminator

Attribute array it87_attributes_in lacks its NULL terminator,
causing random behavior when operating on the attribute group.

Fixes: 52929715

 ("hwmon: (it87) Use is_visible for voltage sensors")
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>