Update MPD to 0.21.25

Source code

ver 0.21.25 (2020/07/06)

• protocol:
  • fix crash when using "rangeid" while playing
• database
  • simple: automatically scan new mounts
  • upnp: fix compatibility with Plex DLNA
• storage
  • fix disappearing mounts after mounting twice
  • udisks: fix reading ".mpdignore"
• input
  • file: detect premature end of file
  • smbclient: don't send credentials to MPD clients
• decoder
  • opus: apply pre-skip and end trimming
  • opus: fix memory leak
  • opus: fix crash bug
  • vorbis: fix crash bug
• output
  • osx: improve sample rate selection
  • osx: fix noise while stopping
• neighbor
  • upnp: fix crash during shutdown
• Windows/Android:
  • fix Boost detection after breaking change in Meson 0.54