Add 'mate' limit to 'bench' command

It is now possible to run SF on a 'mate in x' testsuite.

For instance in case of a file with fen strings of
positions with mate in 10 we can now 'bench' on it:

stockfish bench 128 1 10 mate_in_10.epd mate

No functional change.

src/benchmark.cpp

@@ -82,6 +82,9 @@ void benchmark(const Position& current, istream& is) {
   else if (limitType == "nodes")
       limits.nodes = atoi(limit.c_str());
 
+  else if (limitType == "mate")
+      limits.mate = atoi(limit.c_str());
+
   else
       limits.depth = atoi(limit.c_str());