fix building of FLTK widget opcodes with VS
fix building of FLTK widget opcodes with VS

nikhilsinghmus/csound,nikhilsinghmus/csound,nikhilsinghmus/csound,nikhilsinghmus/csound,nikhilsinghmus/csound,nikhilsinghmus/csound,nikhilsinghmus/csound,nikhilsinghmus/csound,nikhilsinghmus/csound,nikhilsinghmus/csound