added missing destructors to abstract classes that clang 6 complains about

library/SolverUtils/Advection/Advection.h

@@ -69,6 +69,10 @@ typedef std::function<void (
 class Advection
 {
 public:
+
+    SOLVER_UTILS_EXPORT virtual ~Advection()
+    {};
+
     /// Interface function to initialise the advection object.
     SOLVER_UTILS_EXPORT void InitObject(
         LibUtilities::SessionReaderSharedPtr               pSession,

library/SolverUtils/Diffusion/Diffusion.h

@@ -71,6 +71,10 @@ namespace Nektar
         class Diffusion
         {
         public:
+
+            SOLVER_UTILS_EXPORT virtual ~Diffusion()
+            {};
+
             SOLVER_UTILS_EXPORT void InitObject(
                 LibUtilities::SessionReaderSharedPtr              pSession,
                 Array<OneD, MultiRegions::ExpListSharedPtr>       pFields);

library/SolverUtils/RiemannSolvers/RiemannSolver.h

@@ -160,6 +160,9 @@ namespace Nektar
             SOLVER_UTILS_EXPORT RiemannSolver(
                 const LibUtilities::SessionReaderSharedPtr& pSession);
 
+            SOLVER_UTILS_EXPORT virtual ~RiemannSolver()
+            {};
+
             virtual void v_Solve(
                 const int                                         nDim,
                 const Array<OneD, const Array<OneD, NekDouble> > &Fwd,