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_fast.c b/xmss_fast.c
index b918f13..5e3a543 100644
--- a/xmss_fast.c
+++ b/xmss_fast.c
@@ -1,3 +1,4 @@
+#ifdef WITH_XMSS
 /*
 xmss_fast.c version 20160722
 Andreas Hülsing
@@ -1101,3 +1102,4 @@
   *msglen = -1;
   return -1;
 }
+#endif /* WITH_XMSS */