Remove device_tree.o from hw/moxie/Makefile.objs.

Here's a fix for the build problem identified by Aurelien Jarno here:
http://lists.gnu.org/archive/html/qemu-devel/2013-03/msg04177.html

Signed-off-by: Anthony Green <green@moxielogic.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>