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>

include/xen/interface/version.h

@@ -63,6 +63,9 @@ struct xen_feature_info {
 /* arg == xen_domain_handle_t. */
 #define XENVER_guest_handle 8
 
+/* Declares the xen_domain() macros. */
+#include <xen/xen.h>
+
 /* Check if running on Xen version (major, minor) or later */
 static inline bool
 xen_running_on_version_or_later(unsigned int major, unsigned int minor)