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

Index: playlist.c
--- playlist.c.orig
+++ playlist.c
@@ -846,7 +846,7 @@ void plist_shuffle (struct plist *plist)
 	int i;
 
 	for (i = 0; i < plist->num; i += 1)
-		plist_swap (plist, i, (rand () / (float)RAND_MAX) * (plist->num - 1));
+		plist_swap (plist, i, arc4random_uniform(plist->num));
 
 	rb_tree_clear (plist->search_tree);
 
