fixed: put variable under ifdef to avoid unused warnings without mpi

opm/autodiff/BlackoilModelBase_impl.hpp

@@ -228,9 +228,9 @@ namespace detail {
 
         // TODO: put this for now to avoid modify the following code.
         // TODO: this code can be fragile.
+#if HAVE_MPI
         const Wells* wells_arg = asImpl().well_model_.wellsPointer();
 
-#if HAVE_MPI
         if ( linsolver_.parallelInformation().type() == typeid(ParallelISTLInformation) )
         {
             const ParallelISTLInformation& info =