Add constexpr to constructor of FieldVector<K,1>

Otherwise FieldVector<K,1> is not literal and its size()
method can never be used as constexpr.

dune/common/fvector.hh

@@ -224,7 +224,7 @@ namespace Dune {
     //===== construction
 
     /** \brief Default constructor */
-    FieldVector ()
+    DUNE_CONSTEXPR FieldVector ()
       : _data()
     {}