Fix a couple of warnings under Intel compiler

No functional change.

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

src/material.h

@@ -59,7 +59,7 @@ public:
   Value evaluate(const Position& pos) const;
 
 private:
-  void clear();
+  inline void clear();
 
   Key key;
   int16_t mgValue;

src/pawns.h

@@ -59,7 +59,7 @@ public:
   void setKingShelter(Color c, Square ksq, int value);
 
 private:
-  void clear();
+  inline void clear();
 
   Key key;
   Bitboard passedPawns;