Types that are first encountered in declarations such as this interpreted
as living in the enclosing namespace. To solve the problem, forward declare
the type:

namespace mupdf {

    public class Data
    {
    public:
          struct fz_stream_s;
        typedef struct fz_stream_s fz_stream;

        struct fz_stream_s
        {
            int refs;
            int kind;
            int mode;
            int dead;
            fz_buffer *buffer;
            fz_filter *filter;
            fz_stream *chain;
            fz_error *error;
            int file;
        };
       };
}

This tells the compiler that fz_stream_s is a member of Data and not the
enclosing namespace. That said, I don't know why you don't go with the
simpler:

namespace mupdf {

    public class Data
    {
    public:

        struct fz_stream
        {
            int refs;
            int kind;
            int mode;
            int dead;
            fz_buffer *buffer;
            fz_filter *filter;
            fz_stream *chain;
            fz_error *error;
            int file;
        };
       };
}