/usr/include/Swiften/Client/ is in libswiften-dev 2.0+dev6-1+b1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/Swiften/Client/BlockList.h
/usr/include/Swiften/Client/BlockListImpl.h
/usr/include/Swiften/Client/Client.h
/usr/include/Swiften/Client/ClientBlockListManager.h
/usr/include/Swiften/Client/ClientError.h
/usr/include/Swiften/Client/ClientOptions.h
/usr/include/Swiften/Client/ClientSession.h
/usr/include/Swiften/Client/ClientSessionStanzaChannel.h
/usr/include/Swiften/Client/ClientXMLTracer.h
/usr/include/Swiften/Client/CoreClient.h
/usr/include/Swiften/Client/DummyNickManager.h
/usr/include/Swiften/Client/DummyStanzaChannel.h
/usr/include/Swiften/Client/MemoryStorages.h
/usr/include/Swiften/Client/NickManager.h
/usr/include/Swiften/Client/NickManagerImpl.h
/usr/include/Swiften/Client/NickResolver.h
/usr/include/Swiften/Client/StanzaChannel.h
/usr/include/Swiften/Client/Storages.h
/usr/include/Swiften/Client/XMLBeautifier.h