add missing 'const'

[[Imported from SVN: r1594]]

dune/istl/vbvector.hh

@@ -637,7 +637,7 @@ namespace Dune {
 
     //! @returns an iterator that is positioned before
     //! the first entry of the vector.
-    Iterator beforeBegin ()
+    Iterator beforeBegin () const
     {
       return Iterator(block,-1);
     }