Explicitly include boost/filesystem/fstream.hpp to fix missing template
declaration for boost::filesystem::ofstream.

Index: src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine_test.cpp
--- src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine_test.cpp.orig
+++ src/mongo/db/storage/wiredtiger/wiredtiger_kv_engine_test.cpp
@@ -35,6 +35,7 @@
 
 #include <boost/filesystem.hpp>
 #include <boost/filesystem/path.hpp>
+#include <boost/filesystem/fstream.hpp>
 #include <memory>
 
 #include "mongo/base/init.h"
