fix couple of gcc4 warnings by adding (trivial) virtual destructors

Mesh/BDS.h

@@ -69,6 +69,7 @@ public:
 class BDS_Surface
 {
 public :
+  virtual ~BDS_Surface(){}
   virtual double signedDistanceTo(double x, double y, double z) const = 0;
   virtual void projection(double xa, double ya, double za,
 			  double &x, double &y, double &z) const =0;

Mesh/SwapPatterns.h

@@ -122,6 +122,7 @@ protected :
   virtual void GetTriangleWithoutRot(int iPattern, int iTriangle,int tri[3])=0;
 public :
   SwapPattern(){Rot = 0;}
+  virtual ~SwapPattern(){}
   int GetNbPatterns() {return NbPatterns;}
   virtual int GetNbRotations(int iPattern) = 0;
   int GetNbTriangles(){return NbNod-2;}