openfst: 1.6.3 -> 1.6.6

Semi-automatic update. These checks were done:

- built on NixOS
- ran `/nix/store/4yq76v4x9kkmwqmvrcxh89kh1lp96gg0-openfst-1.6.6/bin/fstcompile help` got 0 exit code
- found 1.6.6 with grep in /nix/store/4yq76v4x9kkmwqmvrcxh89kh1lp96gg0-openfst-1.6.6
- found 1.6.6 in filename of file in /nix/store/4yq76v4x9kkmwqmvrcxh89kh1lp96gg0-openfst-1.6.6