Make sure code compiles on Ubuntu 12.04 with boost 1.46

Work with permissions in boost::filesystem was introduced in boost 1.49
diff --git a/src/fetcher.h b/src/fetcher.h
index 185e2a1..515b2e9 100644
--- a/src/fetcher.h
+++ b/src/fetcher.h
@@ -28,6 +28,7 @@
 #include "executor.h"
 #include <boost/intrusive/list.hpp>
 #include <boost/date_time/posix_time/posix_time_types.hpp>
+#include <set>
 
 #include <set>