mpd: Patch mpd to deal with format function being const in fmt-11.0.0 onwards

mpd: Patch mpd to deal with format function being const in fmt-11.0.0 onwards

- Commit has been made in mpd but no release has yet been made with the change. When the
   next version release of mpd occurs this patch can be removed.
- The patch changes all format calls to be const . Without this patch mpd will not build
   with fmt-11.0.0 or newer.
- Update of rootfile not required.

Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>