Avoid warning about implicit malloc()

diff -r 587fa6195b61 lib/imagelib.c
--- a/lib/imagelib.c	Mon Oct 05 22:08:52 2020 +0000
+++ b/lib/imagelib.c	Mon Oct 05 22:09:12 2020 +0000
@@ -1,3 +1,4 @@
+#include <stdlib.h>
 #include <stdio.h>
 #include <string.h>
 #include "imagelib.h"