Log Message:
fix the build since fmtlib 11 import.

taken from upstream commit:
   \ 
https://github.com/MusicPlayerDaemon/MPD/commit/1402869715e3efca87942d79c3173a6b21a6925d

lib/fmt: support build with libfmt-11.0.0

Upstream libfmt commit fmtlib/fmt@d707292
now requires the format function to be const.

Adjust the function prototype so it is const and can compile.

Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>