Conditionally compile XMSS code.

The XMSS code is currently experimental and, unlike the rest of OpenSSH
cannot currently be compiled with a c89 compiler.
diff --git a/sshkey-xmss.c b/sshkey-xmss.c
index f146098..e2b7c49 100644
--- a/sshkey-xmss.c
+++ b/sshkey-xmss.c
@@ -1,3 +1,4 @@
+#ifdef WITH_XMSS
 /* $OpenBSD: sshkey-xmss.c,v 1.1 2018/02/23 15:58:38 markus Exp $ */
 /*
  * Copyright (c) 2017 Markus Friedl.  All rights reserved.
@@ -1051,3 +1052,4 @@
 	state->maxidx = state->idx + maxsign;
 	return 0;
 }
+#endif /* WITH_XMSS */