Fix static_cast to const ref.

dune/matrix-vector/singlenonzerocolumnmatrix.hh

@@ -96,7 +96,7 @@ public:
 
     const_row_reference operator[] (size_type rowIndex) const
     {
-        return const_row_reference(&(static_cast<K&>(nonZeroColumn_[rowIndex])), columnIndex_);
+        return const_row_reference(&(static_cast<const K&>(nonZeroColumn_[rowIndex])), columnIndex_);
     }
 
     size_type nonZeroColumnIndex() const