xen: sysfs: include err.h for PTR_ERR etc

Fixes build error on ARM:
drivers/xen/sys-hypervisor.c: In function 'uuid_show_fallback':
drivers/xen/sys-hypervisor.c:127:2: error: implicit declaration of function 'IS_ERR' [-Werror=implicit-function-declaration]
drivers/xen/sys-hypervisor.c:128:3: error: implicit declaration of function 'PTR_ERR' [-Werror=implicit-function-declaration]

Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>