NewtonIterationBlackoilInterface: make its destructor virtual

this prevents GCC-7's address sanitizer from complaining about a
delete for a new of a different type. (i.e., the new is for the
derived class, but if the destructor is non-virtual only the dtor of
the base class is called if a pointer to the base class is deleted.)

finding this was quite some fun, mainly because it took a while what
the issue was.

opm/autodiff/NewtonIterationBlackoilInterface.hpp

@@ -34,6 +34,8 @@ namespace Opm
         /// Return type for linearSolve(). A simple, non-ad vector type.
         typedef LinearisedBlackoilResidual::ADB::V SolutionVector;
 
+        virtual ~NewtonIterationBlackoilInterface() {}
+
         /// Solve the linear system Ax = b, with A being the
         /// combined derivative matrix of the residual and b
         /// being the residual itself.