Log Message:
musicpd: update to 0.21.8.

ver 0.21.8 (2019/04/23)
* input
  - smbclient: download to buffer instead of throttling transfer
* output
  - httpd: add missing mutex lock
  - httpd: fix use-after-free bug
* playlist
  - soundcloud: fix "Unsupported URI scheme" (0.21.6 regression)
* fix Bonjour bug
* fix build failure with GCC 9
* fix build failure with -Ddatabase=false
* systemd: add user socket unit
* doc: "list file" is deprecated