Don't use malloc.h header on OpenBSD.

--- src/extrude.c.orig	Wed Nov 19 22:58:41 2014
+++ src/extrude.c	Wed Nov 19 22:58:47 2014
@@ -16,7 +16,6 @@
  */
 
 
-#include <malloc.h>
 #include <math.h>
 #include <stdlib.h>
 #include <string.h>	/* for the memcpy() subroutine */
