Rearrange some headers to make sure that out nbase errno defines are seen early,

avoiding warning about symbol redefinitions. This is mostly moving "nmap.h" to
the top of the list.

xml.cc

@@ -145,14 +145,15 @@ All writing is done with log_write(LOG_XML), so if LOG_XML hasn't been
 opened, calling these functions has no effect.
 */
 
+#include "nmap.h"
+#include "output.h"
+#include "xml.h"
+
 #include <assert.h>
 #include <stdarg.h>
 #include <stdio.h>
 #include <list>
 
-#include "output.h"
-#include "xml.h"
-
 struct xml_writer {
   /* Sanity checking: Don't open a new tag while still defining
      attributes for another, like "<elem1<elem2". */