Don't use malloc.h header on OpenBSD.

--- src/texgen.c.orig	Wed Nov 19 23:00:04 2014
+++ src/texgen.c	Wed Nov 19 23:00:10 2014
@@ -13,7 +13,6 @@
  * Copyright (c) 1994,1995 Linas Vepstas <linas@linas.org>
  */
 
-#include <malloc.h>
 #include <math.h>
 #include <stdlib.h>
 
