Use correct type

[[Imported from SVN: r1447]]

dune/istl/owneroverlapcopy.hh

@@ -284,7 +284,7 @@ namespace Dune {
      * @return The Solver Category.
      */
 
-    const SolverCategory getSolverCategory () const {
+    const SolverCategory::Category getSolverCategory () const {
       return category;
     }