Hide global visibility when not needed

Also move PieceValue definition in psqt.cpp,
where it is initialized.

Fix a warning in popcount16() with Intel compiler

No functional change.

src/position.cpp

@@ -34,10 +34,6 @@
 
 using std::string;
 
-Value PieceValue[PHASE_NB][PIECE_NB] = {
-{ VALUE_ZERO, PawnValueMg, KnightValueMg, BishopValueMg, RookValueMg, QueenValueMg },
-{ VALUE_ZERO, PawnValueEg, KnightValueEg, BishopValueEg, RookValueEg, QueenValueEg } };
-
 namespace Zobrist {
 
   Key psq[COLOR_NB][PIECE_TYPE_NB][SQUARE_NB];