mpd_clientlib: 2.13 -> 2.14

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 2.14 with grep in /nix/store/8q00s6prlgljs1imp7r40rq3jyafzlhc-libmpdclient-2.14
- found 2.14 in filename of file in /nix/store/8q00s6prlgljs1imp7r40rq3jyafzlhc-libmpdclient-2.14
- directory tree listing: https://gist.github.com/acd3295fdde722768ff964a7284a00c4