Use a std::vector to store searchMoves

A std::set (that is a rb_tree) seems really
overkill to store at most a handful of moves
and nothing in the common case.

No functional change.

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

src/benchmark.cpp

@@ -118,7 +118,7 @@ void benchmark(istringstream& is) {
       }
       else
       {
-          Threads.start_searching(pos, limits);
+          Threads.start_searching(pos, limits, vector<Move>());
           Threads.wait_for_search_finished();
           nodes += Search::RootPosition.nodes_searched();
       }