Fix a compile bug of patch 18163 (5fbcbee355e2)

Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>

diff -r 39cb0de3ee2f xen/arch/x86/mm/shadow/multi.c
--- a/xen/arch/x86/mm/shadow/multi.c	Tue Jul 29 17:00:29 2008 +0100
+++ b/xen/arch/x86/mm/shadow/multi.c	Wed Jul 30 06:33:55 2008 +0800
@@ -4645,7 +4645,9 @@ static void *emulate_map_dest(struct vcp
                               u32 bytes,
                               struct sh_emulate_ctxt *sh_ctxt)
 {
+#ifndef NDEBUG
     struct segment_register *sreg;
+#endif
     unsigned long offset;
     void *map = NULL;