No-MPI build with PETSC backend fixed

src/amdis/linearalgebra/petsc/MatrixNnzStructure.hpp

@@ -36,7 +36,9 @@ namespace AMDiS
     unsigned long changeIndex_ = 0;
     bool initialized_ = false;
 
+#if HAVE_MPI
     const Mpi::Tag tag_{916821}; //< Communication tag used internally
+#endif
   };
 
 } // end namespace AMDiS