NNUE namespace cleanup

This patch moves the nnue namespace in the appropiate header that correspondes with the definition.
It also makes navigation a bit easier.

closes https://github.com/official-stockfish/Stockfish/pull/4445

No functional change

src/search.cpp

@@ -34,6 +34,7 @@
 #include "tt.h"
 #include "uci.h"
 #include "syzygy/tbprobe.h"
+#include "nnue/evaluate_nnue.h"
 
 namespace Stockfish {