Mark IFormula destructor as virtual

This avoids compile warnings and ensures correct behaviour for future
IFormula subclasses implementing a destructor.

GMLMIP-0.1/formulas/formula.h

@@ -41,6 +41,7 @@ class IFormula {
 		virtual bdd modal(bdd *b, int n, int m)=0;
 		virtual bool satisfiability(int option)=0;
 		virtual void clear_maps()=0;
+		virtual ~IFormula() { };
 };
 
 template<class ModalValueType>