japanese/fcitx5-hazkey: Unbreak build with clang 21

The same workaroundas for lang/swift6, defining
_WCHAR_H_CPLUSPLUS_98_CONFORMANCE_ in c++ compiler flags, works.

PR: 296798
Reported by: Tomoaki AOKI <junchoon@dec.sakura.ne.jp>
Tested by: Tomoaki AOKI <junchoon@dec.sakura.ne.jp>