libchewing: 0.4.0 -> 0.5.1

Semi-automatic update. These checks were performed:

- built on NixOS
- found 0.5.1 with grep in /nix/store/j3cl2l9f1m6p14b5y8cf24p4hqhw9nsn-libchewing-0.5.1
- found 0.5.1 in filename of file in /nix/store/j3cl2l9f1m6p14b5y8cf24p4hqhw9nsn-libchewing-0.5.1

cc "@ericsagnes"