japanese/fcitx5-skk: Depend on ECM unconditionally

The top-level CMakeLists.txt calls find_package(ECM 1.0.0 REQUIRED)
unconditionally, but the port only declared the dependency under the QT6
option, so configure fails with WITHOUT_QT6. Move it out of the option
and make it build-only. ECM is listed in _USE_KDE_LATEST, so USES=kde:6
can be dropped along with it, as done in 95f9a6808973 for the other
fcitx5 ports.

PR: 297622
Approved by: osa (mentor)