converters/pdf2djvu: Mark port as BROKEN and DEPRECATED for removal

Port does not compile anymore with either LLVM or GCC. It has been not been
updated upstream for years and officially abandoned: https://groups.io/g/pdf2djvu/message/146
Unless someone can fix the C++ code this port will be removed.

PR: 287655
Approved by: jrm (mentor)
Differential Revision: https://reviews.freebsd.org/D51004