OtherCommands: re-add the "volume" command

This command was removed by commit 206392ad (MPD 0.16), even though it
was been proven useful for some very simple clients.  On request, I
add it to the protocol again.

src/command/OtherCommands.hxx

@@ -51,6 +51,9 @@ handle_rescan(Client &client, int argc, char *argv[]);
 CommandResult
 handle_setvol(Client &client, int argc, char *argv[]);
 
+CommandResult
+handle_volume(Client &client, int argc, char *argv[]);
+
 CommandResult
 handle_stats(Client &client, int argc, char *argv[]);