[solvers.hh] Always convert the reduction to double explicitly.

This already happened for most solvers when setting the reduction in the
result struct.  It is may be needed when the field type is some extended
number type that prohibits implicit conversion to `double`.  This commit
brings the remaining two solvers in line that did not do this yet.

dune/istl/solvers.hh

@@ -127,7 +127,7 @@ namespace Dune {
 
       // fill statistics
       res.iterations = i;
-      res.reduction = max_value(def/def0);
+      res.reduction = static_cast<double>(max_value(def/def0));
       res.conv_rate  = pow(res.reduction,1.0/i);
       res.elapsed = watch.elapsed();
 
@@ -1512,7 +1512,7 @@ namespace Dune {
 
       // fill statistics
       res.iterations = i;
-      res.reduction = max_value(def/def0);
+      res.reduction = static_cast<double>(max_value(def/def0));
       res.conv_rate  = pow(res.reduction,1.0/i);
       res.elapsed = watch.elapsed();