Build XEN PV without WITNESS support

Hi list,

building a XEN pv domU without WITNESS support, fails with the following
error:

In file included from /usr/src/sys/dev/xen/netfront/netfront.c:33:
/usr/src/sys/sys/sx.h:210:2: error: #error "LOCK_DEBUG not defined,
include <sys/lock.h> before <sys/sx.h>"
mkdep: compile failed
*** Error code 1

Stop in /usr/obj/usr/src/sys/freebsd8_WITHOUT_WITNESS.
*** Error code 1


The bug has been acknowledged by Kip Macy in the beginning of february,
but unfortunately it's not fixed in SVN yet.

The appended patch does the fixing. 


Regards,
Olli