/usr/include/fcitx/ is in fcitx-libs-dev 1:4.2.0-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/fcitx-config/fcitx-config.h
/usr/include/fcitx-config/hotkey.h
/usr/include/fcitx-config/xdg.h
/usr/include/fcitx-utils/log.h
/usr/include/fcitx-utils/memory.h
/usr/include/fcitx-utils/utarray.h
/usr/include/fcitx-utils/utf8.h
/usr/include/fcitx-utils/uthash.h
/usr/include/fcitx-utils/utils.h
/usr/include/fcitx/addon.h
/usr/include/fcitx/candidate.h
/usr/include/fcitx/configfile.h
/usr/include/fcitx/context.h
/usr/include/fcitx/fcitx.h
/usr/include/fcitx/frontend.h
/usr/include/fcitx/hook.h
/usr/include/fcitx/ime.h
/usr/include/fcitx/instance.h
/usr/include/fcitx/keys.h
/usr/include/fcitx/module/
/usr/include/fcitx/module.h
/usr/include/fcitx/module/chttrans/
/usr/include/fcitx/module/chttrans/chttrans.h
/usr/include/fcitx/module/classicui/
/usr/include/fcitx/module/classicui/classicuiinterface.h
/usr/include/fcitx/module/dbus/
/usr/include/fcitx/module/dbus/dbusstuff.h
/usr/include/fcitx/module/ipc/
/usr/include/fcitx/module/ipc/ipc.h
/usr/include/fcitx/module/pinyin/
/usr/include/fcitx/module/pinyin/pydef.h
/usr/include/fcitx/module/punc/
/usr/include/fcitx/module/punc/punc.h
/usr/include/fcitx/module/x11/
/usr/include/fcitx/module/x11/x11stuff.h
/usr/include/fcitx/profile.h
/usr/include/fcitx/ui.h