playlist: moved PlaylistInfo() to playlist_print.c

PlaylistInfo() (notice the capital 'P') sends a stored playlist to the
client.  Move it to a separate library, where all the code which glues
the playlist and the MPD protocol together will live.

src/command.c

@@ -19,6 +19,7 @@
 #include "command.h"
 #include "player_control.h"
 #include "playlist.h"
+#include "playlist_print.h"
 #include "ls.h"
 #include "directory.h"
 #include "directory_print.h"