musicpd: update to 0.20.21.

ver 0.20.21 (2018/08/17)
* database
  - proxy: add "password" setting
  - proxy: support tags "ArtistSort", "AlbumArtistSort", "AlbumSort"
  - simple: allow .mpdignore comments only at start of line
* output
  - httpd: remove broken DLNA support code
* playlist
  - cue: support file type declaration "FLAC" (non-standard)
* URI schemes are case insensitive
* Android, Windows
  - enable the "curl" storage plugin

(wiz)