Minor - fixup missing "extern" declaration in xen.h.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/xen.h b/src/xen.h
index cc506a6..b664d9c 100644
--- a/src/xen.h
+++ b/src/xen.h
@@ -17,7 +17,7 @@
     return (xen_cpuid_base != 0);
 }
 
-unsigned long xen_hypercall_page;
+extern unsigned long xen_hypercall_page;
 
 #define _hypercall0(type, name)                                         \
 ({                                                                      \