xen/acpi: Fix compile error by missing decleration for xen_domain.

Commit 92e3229d
("xen/acpi: ACPI PAD driver") adds a new function but forgets to
use the right header. Without it, we get:

In file included from drivers/xen/features.c:15:0:
include/xen/interface/version.h: In function ‘xen_running_on_version_or_later’:
include/xen/interface/version.h:72:2: error: implicit declaration of function ‘xen_domain’ [-Werror=implicit-function-declaration]

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>