app-i18n/fcitx-libpinyin[dictmanager] depends on dev-qt/qtwebkit:4 which is ancient and will never see any security updates.
If there's no upstream fix available I'd suggest removing the USE flag.
app-i18n/fcitx-libpinyin-0.5.1 is now in the tree.
It uses Qt 5 (including QtWebEngine) instead of Qt 4.
Versions <app-i18n/fcitx-libpinyin-0.5.0 are still in tree and need to be cleaned up or will otherwise be masked and dropped back to ~arch.
For the time being, <app-i18n/fcitx-libpinyin-0.5.0[dictionary-manager] was masked, as discussed.