Merge #14768: revert removal of fstream.hpp header in fs.h

e816b34 revert removal of fstream.hpp header in fs.h (Karl-Johan Alm)

Pull request description:

  We cannot (yet) remove the EXPECTED_BOOST_INCLUDES entry as this header is still needed in `fs.h` (see #14763).

  Partially reverts #14718.

Tree-SHA512: e94d8d6208bee14af20a7a529e60a4898358ec8c070a8bf0701e589a2ae33df1305deac83cee619f103c24be0eb3c12a2f490209c125b247acf21561c7de456e

Author: laanwj

src/fs.h

@@ -12,6 +12,7 @@
 #endif
 
 #include <boost/filesystem.hpp>
+#include <boost/filesystem/fstream.hpp>
 
 /** Filesystem operations and types */
 namespace fs = boost::filesystem;