/usr/include/mjpegtools/mplex/ is in libmjpegtools-dev 1:2.1.0+debian-4.
This file is owned by root:root, with mode 0o755.
..
/usr/include/mjpegtools/mplex/audiostrm.hpp
/usr/include/mjpegtools/mplex/aunit.hpp
/usr/include/mjpegtools/mplex/aunitbuffer.hpp
/usr/include/mjpegtools/mplex/bits.hpp
/usr/include/mjpegtools/mplex/decodebufmodel.hpp
/usr/include/mjpegtools/mplex/inputstrm.hpp
/usr/include/mjpegtools/mplex/interact.hpp
/usr/include/mjpegtools/mplex/mplexconsts.hpp
/usr/include/mjpegtools/mplex/multiplexor.hpp
/usr/include/mjpegtools/mplex/outputstrm.hpp
/usr/include/mjpegtools/mplex/padstrm.hpp
/usr/include/mjpegtools/mplex/stillsstream.hpp
/usr/include/mjpegtools/mplex/stream_params.hpp
/usr/include/mjpegtools/mplex/systems.hpp
/usr/include/mjpegtools/mplex/videostrm.hpp