Include stdlib.h for declaration of free.

Fixes build with -Werror on at least Fedora and probably others.
diff --git a/openbsd-compat/freezero.c b/openbsd-compat/freezero.c
index 90b9d38..bad018f 100644
--- a/openbsd-compat/freezero.c
+++ b/openbsd-compat/freezero.c
@@ -16,6 +16,7 @@
 
 #include "includes.h"
 
+#include <stdlib.h>
 #include <string.h>
 
 #ifndef HAVE_FREEZERO