provide directory.hpp for directory_iterator

Index: external/s25edit/callbacks.cpp
--- external/s25edit/callbacks.cpp.orig
+++ external/s25edit/callbacks.cpp
@@ -18,6 +18,7 @@
 #include "CSurface.h"
 #include "globals.h"
 #include "helpers/format.hpp"
+#include <boost/filesystem/directory.hpp>
 #include <boost/filesystem/operations.hpp>
 #include <boost/filesystem/path.hpp>
 #include <algorithm>
