Added missing virtual destructors to LinearSolver and Preconditioner

src/TNL/Solvers/Linear/LinearSolver.h

@@ -60,6 +60,8 @@ public:
 
    virtual bool solve( ConstVectorViewType b, VectorViewType x ) = 0;
 
+   virtual ~LinearSolver() {}
+
 protected:
    MatrixPointer matrix = nullptr;
    PreconditionerPointer preconditioner = nullptr;

src/TNL/Solvers/Linear/Preconditioners/Preconditioner.h

@@ -52,6 +52,8 @@ public:
    {
       return String( "Preconditioner" );
    }
+
+   virtual ~Preconditioner() {}
 };
 
 } // namespace Preconditioners