/usr/include/dlib/threads/ is in libdlib-dev 18.18-2build1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/dlib/threads/auto_mutex_extension.h
/usr/include/dlib/threads/auto_mutex_extension_abstract.h
/usr/include/dlib/threads/auto_unlock_extension.h
/usr/include/dlib/threads/auto_unlock_extension_abstract.h
/usr/include/dlib/threads/create_new_thread_extension.h
/usr/include/dlib/threads/create_new_thread_extension_abstract.h
/usr/include/dlib/threads/multithreaded_object_extension.h
/usr/include/dlib/threads/multithreaded_object_extension_abstract.h
/usr/include/dlib/threads/parallel_for_extension.h
/usr/include/dlib/threads/parallel_for_extension_abstract.h
/usr/include/dlib/threads/posix.h
/usr/include/dlib/threads/read_write_mutex_extension.h
/usr/include/dlib/threads/read_write_mutex_extension_abstract.h
/usr/include/dlib/threads/rmutex_extension.h
/usr/include/dlib/threads/rmutex_extension_abstract.h
/usr/include/dlib/threads/rsignaler_extension.h
/usr/include/dlib/threads/rsignaler_extension_abstract.h
/usr/include/dlib/threads/thread_function_extension.h
/usr/include/dlib/threads/thread_function_extension_abstract.h
/usr/include/dlib/threads/thread_pool_extension.h
/usr/include/dlib/threads/thread_pool_extension_abstract.h
/usr/include/dlib/threads/thread_specific_data_extension.h
/usr/include/dlib/threads/thread_specific_data_extension_abstract.h
/usr/include/dlib/threads/threaded_object_extension.h
/usr/include/dlib/threads/threaded_object_extension_abstract.h
/usr/include/dlib/threads/threads_kernel.h
/usr/include/dlib/threads/threads_kernel_1.h
/usr/include/dlib/threads/threads_kernel_2.h
/usr/include/dlib/threads/threads_kernel_abstract.h
/usr/include/dlib/threads/threads_kernel_shared.h
/usr/include/dlib/threads/windows.h