Add Stockfish namespace.

fixes #3350 and is a small cleanup that might make it easier to use SF
in separate projects, like a NNUE trainer or similar.

closes https://github.com/official-stockfish/Stockfish/pull/3370

No functional change.

src/material.h

@@ -24,7 +24,7 @@
 #include "position.h"
 #include "types.h"
 
-namespace Material {
+namespace Stockfish::Material {
 
 /// Material::Entry contains various information about a material configuration.
 /// It contains a material imbalance evaluation, a function pointer to a special
@@ -66,6 +66,6 @@ typedef HashTable<Entry, 8192> Table;
 
 Entry* probe(const Position& pos);
 
-} // namespace Material
+} // namespace Stockfish::Material
 
 #endif // #ifndef MATERIAL_H_INCLUDED