/usr/include/flann/algorithms/ is in libflann-dev 1.9.1+dfsg-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/flann/algorithms/all_indices.h
/usr/include/flann/algorithms/autotuned_index.h
/usr/include/flann/algorithms/center_chooser.h
/usr/include/flann/algorithms/composite_index.h
/usr/include/flann/algorithms/dist.h
/usr/include/flann/algorithms/hierarchical_clustering_index.h
/usr/include/flann/algorithms/kdtree_cuda_3d_index.h
/usr/include/flann/algorithms/kdtree_cuda_builder.h
/usr/include/flann/algorithms/kdtree_index.h
/usr/include/flann/algorithms/kdtree_single_index.h
/usr/include/flann/algorithms/kmeans_index.h
/usr/include/flann/algorithms/linear_index.h
/usr/include/flann/algorithms/lsh_index.h
/usr/include/flann/algorithms/nn_index.h