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/xmss_hash.c b/xmss_hash.c
index c4cac47..c8b2711 100644
--- a/xmss_hash.c
+++ b/xmss_hash.c
@@ -1,3 +1,4 @@
+#ifdef WITH_XMSS
 /*
 hash.c version 20160722
 Andreas Hülsing
@@ -135,3 +136,4 @@
   }
   return core_hash_SHA2(out, 0, key, n, buf, n, n);
 }
+#endif /* WITH_XMSS */