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>

drivers/xen/sys-hypervisor.c

@@ -11,6 +11,7 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/kobject.h>
+#include <linux/err.h>
 
 #include <asm/xen/hypervisor.h>
 #include <asm/xen/hypercall.h>