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_fast.c b/xmss_fast.c
index 5e3a543..577a9c3 100644
--- a/xmss_fast.c
+++ b/xmss_fast.c
@@ -1,4 +1,3 @@
-#ifdef WITH_XMSS
 /*
 xmss_fast.c version 20160722
 Andreas Hülsing
@@ -7,6 +6,7 @@
 */
 
 #include "includes.h"
+#ifdef WITH_XMSS
 
 #include "xmss_fast.h"
 #include <stdlib.h>