Remove signed-unsigned comparison on repartition.hh

to get rid of a compiler warning.

dune/istl/repartition.hh

@@ -787,12 +787,12 @@ namespace Dune
 
     using std::signbit;
     for(Metis::idx_t vtx=0; vtx<(Metis::idx_t)noVtx; ++vtx) {
-      if(xadj[vtx]>noEdges || signbit(xadj[vtx])) {
+      if(static_cast<S>(xadj[vtx])>noEdges || signbit(xadj[vtx])) {
         std::cerr <<"Check graph: xadj["<<vtx<<"]="<<xadj[vtx]<<" (>"
                   <<noEdges<<") out of range!"<<std::endl;
         correct=false;
       }
-      if(xadj[vtx+1]>noEdges || signbit(xadj[vtx+1])) {
+      if(static_cast<S>(xadj[vtx+1])>noEdges || signbit(xadj[vtx+1])) {
         std::cerr <<"Check graph: xadj["<<vtx+1<<"]="<<xadj[vtx+1]<<" (>"
                   <<noEdges<<") out of range!"<<std::endl;
         correct=false;