Log Message:
mpc: update to 0.31.

0.31 (2018/10/24)
* add commands "queued", "seekthrough", "mount", \ 
"unmount"
* support "search" with filter expression (MPD 0.21)
* support "load" with range
* allow only tag names after "list"
* fix the Windows build (no strndup())
* make documentation build optional with -Ddocumentation={true|false|auto}
* build: require Meson 0.47