[cleanup] Encode file path in single-inclusion macro

dune/istl/umfpack.hh

 // -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 // vi: set et ts=4 sw=2 sts=2:
-#ifndef DUNE_UMFPACK_HH
-#define DUNE_UMFPACK_HH
+#ifndef DUNE_ISTL_UMFPACK_HH
+#define DUNE_ISTL_UMFPACK_HH
 
 #if HAVE_UMFPACK
 
@@ -483,4 +483,4 @@ namespace Dune {
 
 #endif //HAVE_UMFPACK
 
-#endif //DUNE_UMFPACK_HH
+#endif //DUNE_ISTL_UMFPACK_HH