Restore Non-MPI Builds

Commit 371b2592f misordered the closing brace of a namespace and
the conditional declarations dependent upon MPI availability.  This
commit restores the expected order and fixes non-MPI builds.

opm/simulators/linalg/ParallelIstlInformation.cpp

@@ -350,6 +350,6 @@ INSTANCE(int)
 INSTANCE(float)
 INSTANCE(std::size_t)
 
-#endif
+} // namespace Opm
 
-}
+#endif // HAVE_MPI && HAVE_DUNE_ISTL