int->size_t; avoid compiler warnings

[[Imported from SVN: r6269]]

dune/solvers/iterationsteps/multigridstep.hh

@@ -149,7 +149,7 @@
         void setTransferOperators(const DerivedTransferHierarchy& transfer)
         {
             mgTransfer_.resize(transfer.size());
-            for(int j=0; j<transfer.size(); ++j)
+            for(size_t j=0; j<transfer.size(); ++j)
                 mgTransfer_[j] = transfer[j];
         }