musicpd: Update to 0.21.16

ver 0.21.16 (2019/10/16)
* queue
  - fix relative destination offset when moving a range
* storage
  - curl: request the "resourcetype" property to fix database update
  - curl: URL-encode more paths
  - curl: follow redirects for collections without trailing slash
* update
  - fix crash when music_directory is not a directory
* fix build with iconv() instead of ICU

(nia)