chinese/fcitx5-chinese-addons: Update to 5.1.15

Add build dependency on devel/nlohmann-json: since 5.1.14,
CMakeLists.txt calls find_package(nlohmann_json 3.2 REQUIRED).

Fix the dependency declarations flagged by stage-qa: boost is only
used for a header (boost/container_hash/hash.hpp), so it is a build
dependency; libfmt is not used at all; the GUI part links against
libFcitx5Qt6WidgetsAddons, not libFcitx5Qt6DBusAddons; and only the
gettext tools (msgfmt) are needed, not libintl.