Fix include

Suggest by @hungte [1][2].

[1] https://github.com/chewing/libchewing/commit/ae9202aa22857c87a8918314f0fcc14be46f68e6#commitcomment-4891531
[2] https://github.com/chewing/libchewing/commit/ae9202aa22857c87a8918314f0fcc14be46f68e6#commitcomment-4891506