xen: add extra pages to balloon

Add extra pages in the pseudo-physical address space to the balloon
so we can extend into them later.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
diff --git a/include/xen/page.h b/include/xen/page.h
index eaf85fa..0be36b9 100644
--- a/include/xen/page.h
+++ b/include/xen/page.h
@@ -1 +1,8 @@
+#ifndef _XEN_PAGE_H
+#define _XEN_PAGE_H
+
 #include <asm/xen/page.h>
+
+extern phys_addr_t xen_extra_mem_start, xen_extra_mem_size;
+
+#endif	/* _XEN_PAGE_H */