smb347-charger: Include missing <linux/err.h>

Without the include we get build errors like:

drivers/power/smb347-charger.c: In function 'smb347_probe':
drivers/power/smb347-charger.c:1039:2: error: implicit declaration of function 'IS_ERR' [-Werror=implicit-function-declaration]
drivers/power/smb347-charger.c:1040:3: error: implicit declaration of function 'PTR_ERR' [-Werror=implicit-function-declaration]

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>