#16 Eliminate ICC remark on return_range struct

[Jason Kankiewicz]

The absence of an access control specifier on the generated pyplusplus::call_policies::return_range struct causes the ICC compiler to output a remark like the following:
.../__return_range.pypp.hpp(55): remark #304: access control not specified ("public" by default)
struct return_range : bpl::default_call_policies{
^

This annoyance was encountered in revision 1856.