Fix 'bench' for Chess960 case

Now a fen file with Chess960 positions is
correctly parsed. But it is mandatory to set
"UCI_Chess960" option _before_ to call bench.

Note that this was not needed/possible before
adding the possibility to call 'bench' from
command prompt.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/benchmark.cpp

@@ -112,7 +112,7 @@ void benchmark(const Position& current, istream& is) {
 
   for (size_t i = 0; i < fens.size(); i++)
   {
-      Position pos(fens[i], false, Threads.main_thread());
+      Position pos(fens[i], Options["UCI_Chess960"], Threads.main_thread());
 
       cerr << "\nPosition: " << i + 1 << '/' << fens.size() << endl;