mosquitto: 1.4.14 -> 1.4.15

Semi-automatic update. 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 1.4.15 with grep in /nix/store/7nifpbj16dlhljb2jwbwxyv4wx1zwa1y-mosquitto-1.4.15
- found 1.4.15 in filename of file in /nix/store/7nifpbj16dlhljb2jwbwxyv4wx1zwa1y-mosquitto-1.4.15