mpc: update to 0.34.

0.34 (2021/11/30)
* add commands "albumart", "readpicture"
* don't print status after error
* custom status format
* support grouping "list" results
* meson: auto-build libmpdclient if not available
* require libmpdclient 2.16 or newer
* require MPD 0.21 or newer

(wiz)