Fix compiler warning on incompatible type

* ibus/engine/engine.c (ibus_lanxang_engine_commit_char):
  - Cast (IBusLanXangEngine *) to (IBusEngine *) as required by
    ibus_engine_commit_text().
Fix GObject warning on non-GObject

* ibus/engine/engine.c (ibus_lanxang_engine_commit_char):
  - Do not unref IBusText object after use. GObject complains it's not a
    GObject.