Fix segfault on clang archs

Index: lib/imagelib.c
--- lib/imagelib.c.orig
+++ lib/imagelib.c
@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #include "imagelib.h"
 #include "cards.h"
