freeswitch: 1.6.15 -> 1.6.19

Semi-automatic update. These checks were done:

- built on NixOS
- ran `/nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19/bin/freeswitch -h` got 0 exit code
- ran `/nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19/bin/fs_cli -h` got 0 exit code
- ran `/nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19/bin/fs_cli --help` got 0 exit code
- ran `/nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19/bin/fs_cli -V` and found version 1.6.19
- ran `/nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19/bin/fs_cli -v` and found version 1.6.19
- ran `/nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19/bin/fs_cli --version` and found version 1.6.19
- ran `/nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19/bin/fs_cli -h` and found version 1.6.19
- ran `/nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19/bin/fs_cli --help` and found version 1.6.19
- ran `/nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19/bin/fsxs --help` got 0 exit code
- found 1.6.19 with grep in /nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19
- found 1.6.19 in filename of file in /nix/store/jpf73ild0rhyvsgngigp3ksb94nmf6pk-freeswitch-1.6.19