Don't use malloc.h header on OpenBSD.

--- src/buffer_event.c.orig	Mon Nov  3 23:32:12 2014
+++ src/buffer_event.c	Mon Nov  3 23:32:26 2014
@@ -3,7 +3,6 @@
 #include "buffer_event.h"
 #include "utility.h"
 #include <lauxlib.h>
-#include <malloc.h>
 #include "event_buffer.h"
 
 #define BUFFER_EVENT_MT "BUFFER_EVENT_MT"