Restore NPS output for Perft

Previously it was possible to also get the node counter after running a bench with perft, i.e.
`./stockfish bench 1 1 5 current perft`, caused by a small regression from the uci refactoring.

```
Nodes searched: 4865609

===========================
Total time (ms) : 18
Nodes searched  : 4865609
Nodes/second    : 270311611
````

closes https://github.com/official-stockfish/Stockfish/pull/5188

No functional change

src/benchmark.h

@@ -23,7 +23,7 @@
 #include <string>
 #include <vector>
 
-namespace Stockfish {
+namespace Stockfish::Benchmark {
 
 std::vector<std::string> setup_bench(const std::string&, std::istream&);