musicpd: update to 0.21.6.

ver 0.21.6 (2019/03/17)
* protocol
  - allow loading playlists specified as absolute filesystem paths
  - fix negated filter expressions with multiple tag values
  - fix "list" with filter expression
  - omit empty playlist names in "listplaylists"
* input
  - cdio_paranoia: fix build failure due to missing #include
* decoder
  - opus: fix replay gain when there are no other tags
  - opus: fix seeking to beginning of song
  - vorbis: fix Tremor conflict resulting in crash
* output
  - pulse: work around error with unusual channel count
  - osx: fix build failure
* playlist
  - flac: fix use-after-free bug
* support abstract sockets on Linux
* Windows
  - remove the unused libwinpthread-1.dll dependency
* Android
  - enable SLES power saving mode

(wiz)