/usr/include/opencv2/flann/ is in libopencv-flann-dev 2.4.8+dfsg1-2ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/opencv2/flann/all_indices.h
/usr/include/opencv2/flann/allocator.h
/usr/include/opencv2/flann/any.h
/usr/include/opencv2/flann/autotuned_index.h
/usr/include/opencv2/flann/composite_index.h
/usr/include/opencv2/flann/config.h
/usr/include/opencv2/flann/defines.h
/usr/include/opencv2/flann/dist.h
/usr/include/opencv2/flann/dummy.h
/usr/include/opencv2/flann/dynamic_bitset.h
/usr/include/opencv2/flann/flann.hpp
/usr/include/opencv2/flann/flann_base.hpp
/usr/include/opencv2/flann/general.h
/usr/include/opencv2/flann/ground_truth.h
/usr/include/opencv2/flann/hdf5.h
/usr/include/opencv2/flann/heap.h
/usr/include/opencv2/flann/hierarchical_clustering_index.h
/usr/include/opencv2/flann/index_testing.h
/usr/include/opencv2/flann/kdtree_index.h
/usr/include/opencv2/flann/kdtree_single_index.h
/usr/include/opencv2/flann/kmeans_index.h
/usr/include/opencv2/flann/linear_index.h
/usr/include/opencv2/flann/logger.h
/usr/include/opencv2/flann/lsh_index.h
/usr/include/opencv2/flann/lsh_table.h
/usr/include/opencv2/flann/matrix.h
/usr/include/opencv2/flann/miniflann.hpp
/usr/include/opencv2/flann/nn_index.h
/usr/include/opencv2/flann/object_factory.h
/usr/include/opencv2/flann/params.h
/usr/include/opencv2/flann/random.h
/usr/include/opencv2/flann/result_set.h
/usr/include/opencv2/flann/sampling.h
/usr/include/opencv2/flann/saving.h
/usr/include/opencv2/flann/simplex_downhill.h
/usr/include/opencv2/flann/timer.h