/usr/include/libktorrent/interfaces/ is in libktorrent-dev 1.1.3-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/libktorrent/interfaces/blocklistinterface.h
/usr/include/libktorrent/interfaces/cachefactory.h
/usr/include/libktorrent/interfaces/chunkdownloadinterface.h
/usr/include/libktorrent/interfaces/chunkselectorinterface.h
/usr/include/libktorrent/interfaces/exitoperation.h
/usr/include/libktorrent/interfaces/logmonitorinterface.h
/usr/include/libktorrent/interfaces/monitorinterface.h
/usr/include/libktorrent/interfaces/peerinterface.h
/usr/include/libktorrent/interfaces/peersource.h
/usr/include/libktorrent/interfaces/piecedownloader.h
/usr/include/libktorrent/interfaces/queuemanagerinterface.h
/usr/include/libktorrent/interfaces/serverinterface.h
/usr/include/libktorrent/interfaces/torrentfileinterface.h
/usr/include/libktorrent/interfaces/torrentinterface.h
/usr/include/libktorrent/interfaces/trackerinterface.h
/usr/include/libktorrent/interfaces/trackerslist.h
/usr/include/libktorrent/interfaces/webseedinterface.h