Add wxBase64Decode(void*,size_t,wxString) overload.

Add wxBase64Decode(void*,size_t,wxString) overload.

We had wxBase64Decode(void*,size_t,const char*,size_t) as well as
wxBase64Decode(const char*,size_t) and wxBase64Decode(wxString) already but
not a version taking the buffer and wxString, complete the set of overloads
now.

This allows the unit test to compile in STL build (where there is no implicit
conversion from wxString to const char *).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62614 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775