fix unused variable d

[eagleoflqj]

/Users/runner/work/fcitx5-ios/fcitx5-ios/engines/libime/src/libime/core/historybigram.cpp:562:5: warning: unused variable 'd' [-Wunused-variable]
  562 |     FCITX_D();
      |     ^~~~~~~~~
/Users/runner/work/fcitx5-ios/fcitx5-ios/fcitx5/src/lib/fcitx-utils/../fcitx-utils/macros.h:21:31: note: expanded from macro 'FCITX_D'
   21 | #define FCITX_D() auto *const d = d_func()
      |                               ^
1 warning generated.