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/bitbase.cpp

@@ -23,6 +23,8 @@
 #include "bitboard.h"
 #include "types.h"
 
+namespace Stockfish {
+
 namespace {
 
   // There are 24 possible pawn squares: files A to D and ranks from 2 to 7.
@@ -66,7 +68,6 @@ namespace {
 
 } // namespace
 
-
 bool Bitbases::probe(Square wksq, Square wpsq, Square bksq, Color stm) {
 
   assert(file_of(wpsq) <= FILE_D);
@@ -96,7 +97,6 @@ void Bitbases::init() {
           KPKBitbase.set(idx);
 }
 
-
 namespace {
 
   KPKPosition::KPKPosition(unsigned idx) {
@@ -168,3 +168,5 @@ namespace {
   }
 
 } // namespace
+
+} // namespace Stockfish