Add WITH_XMSS, move to prevent conflicts.

Add #ifdef WITH_XMSS to ssh-xmss.c, move it in the other files to after
includes.h so it's less likely to conflict and will pick up WITH_XMSS if
added to config.h.
diff --git a/xmss_hash_address.c b/xmss_hash_address.c
index b970619..21ae103 100644
--- a/xmss_hash_address.c
+++ b/xmss_hash_address.c
@@ -1,4 +1,3 @@
-#ifdef WITH_XMSS
 /*
 hash_address.c version 20160722
 Andreas Hülsing
@@ -6,6 +5,7 @@
 Public domain.
 */
 #include "includes.h"
+#ifdef WITH_XMSS
 
 #ifdef HAVE_STDINT_H
 #include <stdint.h>