hwmon: (menf21bmc) Include linux/err.h

Include linux/err.h to get the definitions for IS_ERR() PTR_ERR() and
ERR_PTR() used in the driver.

This fixes compilation on powerpc targets.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@men.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>