Fix clang warning

clang complains about the friend declaration with a warning:

warning : unqualified friend declaration referring to type outside of the nearest enclosing namespace is a Microsoft extension; add a nested name specifier [-Wmicrosoft]
    friend class iterator_core_access;
                       ^
                       ::boost::

Explicitly qualifying the class name with its namespace fixes the warning.
Merge pull request #6 from MarcelRaad/patch-1

Fix clang warning