[cleanup] Fix fallout from changing to C++11's is_convertible

dune/common/iteratorfacades.hh

@@ -6,6 +6,8 @@
 #include <iterator>
 #include <type_traits>
 
+#include "typetraits.hh"
+
 namespace Dune
 {
   /*! \defgroup IteratorFacades Iterator facades

dune/common/test/dummyiterator.hh

@@ -25,7 +25,7 @@ public:
   template<typename T2>
   dummyiterator
     ( const dummyiterator<T2>& o,
-    typename std::enable_if<std::is_convertible<T2&, T&>::exists>::value* = 0)
+    typename std::enable_if<std::is_convertible<T2&, T&>::value>::type* = 0)
     : value(o.value)
   {}