missing err.h header file can cause simillar errors like below
in some configurations.
"error: implicit declaration of function 'ERR_PTR'
[-Werror=implicit-function-declaration]"

This adds the missing include to ensure we can always include
the header.
Signed-off-by: NSrinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>