Call preprocess()

Without this change, steps like ISTLSeqILU0 and ISTLSeqSSOR which
create an underlying ISTL object cannot be used as preconditioners

dune/solvers/iterationsteps/lineariterationstep.hh

@@ -73,6 +73,7 @@ public:
         x = 0;
         this->x_ = &x;
         rhs_     = &r;
+        this->preprocess();
         iterate();
         x = this->getSol();
     }