mpd: bump to version 0.20.9

mpd: bump to version 0.20.9

Building version 0.20.7 fails with a GCC 4.9 toolchain:

```
src/Main.cxx:125:23: error: call to non-constexpr function 'const _Tp& std::max(const _Tp&, const _Tp&) [with _Tp = long unsigned int]'
          64 * KILOBYTE);
                       ^
make[3]: *** [src/Main.o] Error 1
```

The issue was reported upstream [1] and already fixed in version 0.20.8.

Fixes:
http://autobuild.buildroot.net/results/fb6/fb67f453683078023e9d801176ef6871bc720c67/
http://autobuild.buildroot.net/results/200/2005f2a7f2857e59ad1a896db0421bb0328b1766/
http://autobuild.buildroot.net/results/804/80410b62d4ca292df8ecbaf7b0e2a020767111b6/

[1] https://github.com/MusicPlayerDaemon/MPD/issues/45

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>