Fixed some warnings when using -Weffc++ gcc option

Plus some other icc warnings popped up with new and strictier
compile options.

No functional and speed change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/pawns.h

@@ -78,6 +78,9 @@ class PawnInfoTable {
 
   enum SideType { KingSide, QueenSide };
 
+  PawnInfoTable(const PawnInfoTable&);
+  PawnInfoTable& operator=(const PawnInfoTable&);
+
 public:
   PawnInfoTable();
   ~PawnInfoTable();