#13 Eliminate ICC remarks on virtual base class wrappers

[Jason Kankiewicz]

The absence of an access control specifier on a generated struct that wraps a virtual base class causes the ICC compiler to output remarks like the following:
.../VirtualBaseClass.pypp.cpp(...): remark #304: access control not specified ("public" by default)
struct VirtualBaseClass_wrapper : VirtualBaseClass, bp::wrapper< VirtualBaseClass > {
^

Please add a "public" access control specifier to the generated code.

This annoyance was encountered in revision 1856.