Fix for compilers not implementing the resolution to DR45.

[SVN r29699]

include/boost/regex/pending/object_cache.hpp

@@ -51,6 +51,10 @@ private:
       list_type   cont;
       map_type    index;
    };
+
+   // Needed by compilers not implementing the resolution to DR45. For reference,
+   // see http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_defects.html#45.
+   friend struct data;
 };
 
 template <class Key, class Object>