Replace rand(3)-based computation with arc4random_uniform(3).

--- playlist.c.orig	Sun Jun  5 13:54:04 2016
+++ playlist.c	Sun Jun  5 13:58:53 2016
@@ -774,8 +774,7 @@ void plist_shuffle (struct plist *plist)
 	int i;
 
 	for (i = 0; i < plist->num; i++)
-		plist_swap (plist, i,
-				(rand()/(float)RAND_MAX) * (plist->num - 1));
+		plist_swap (plist, i, arc4random_uniform(plist->num));
 
 	rb_clear (&plist->search_tree);