Fix for two-phase name lookup

[[Imported from SVN: r5124]]

dune/solvers/iterationsteps/projectedblockgsstep.cc

@@ -33,7 +33,7 @@ void ProjectedBlockGSStep<OperatorType, DiscFuncType>::iterate()
             continue;
 
         VectorBlock r;
-        residual(i, r);
+        this->residual(i, r);
 
         // Compute x_i += A_{i,i}^{-1} r[i] 
         VectorBlock v;