Fixes a compile error if XOPEN_SOURCE 600 is defined.

See: http://www.mailinglistarchive.com/freebsd-current@freebsd.org/msg15011.html

--- sys/sys/types.h.orig
+++ sys/sys/types.h
@@ -46,7 +46,7 @@
 
 #include <sys/_pthreadtypes.h>
 
-#if __BSD_VISIBLE
+#ifndef _POSIX_SOURCE
 typedef	unsigned char	u_char;
 typedef	unsigned short	u_short;
 typedef	unsigned int	u_int;