Xen: add definition of xen_hypercall_page

commit b7d9abfaeb7d (correctly) added the missing "extern" to the declaration
in xen.h, however it turns out we never actually declared the variable!

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
diff --git a/src/xen.c b/src/xen.c
index 961e316..b18cca2 100644
--- a/src/xen.c
+++ b/src/xen.c
@@ -14,6 +14,7 @@
 #define INFO_PHYSICAL_ADDRESS 0x00001000
 
 u32 xen_cpuid_base = 0;
+unsigned long xen_hypercall_page = 0;
 
 struct xen_seabios_info {
     char signature[14]; /* XenHVMSeaBIOS\0 */