[bugfix,parmetis] Always typedef idxtype.

The type idxtype is used by header repartition even if parmetis is not
used. Therefore it has to be typedefed meaningful in this case,
too. This was not the case before and is accomplished by this commit.

dune/istl/repartition.hh

@@ -778,7 +778,9 @@ namespace Dune
                                   idxtype *adjwgt, int *wgtflag, int *numflag, int *nparts,
                                   int *options, int *edgecut, idxtype *part);
   }
-#endif
+#else
+  typedef std::size_t idxtype;
+#endif // HAVE_PARMETIS
 
   template<class S, class T>
   inline void print_carray(S& os, T* array, std::size_t l)