zebra: fix build on OpenBSD >= 5.9

RTF_XRESOLVE was removed from the OpenBSD tree recently.
diff --git a/zebra/kernel_socket.c b/zebra/kernel_socket.c
index 5e68c56..64c6cbb 100644
--- a/zebra/kernel_socket.c
+++ b/zebra/kernel_socket.c
@@ -242,7 +242,9 @@
 #ifdef RTF_CLONING
   {RTF_CLONING,   "CLONING"},
 #endif /* RTF_CLONING */
+#ifdef RTF_XRESOLVE
   {RTF_XRESOLVE,  "XRESOLVE"},
+#endif /* RTF_XRESOLVE */
 #ifdef RTF_LLINFO
   {RTF_LLINFO,    "LLINFO"},
 #endif /* RTF_LLINFO */