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.htmlSigned-off-by: NAnthony Green <green@moxielogic.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

hw/moxie/Makefile.objs

 # moxie boards
 obj-y = serial.o mc146818rtc.o vga.o
-obj-$(CONFIG_FDT) += device_tree.o
 
 obj-y := $(addprefix ../,$(obj-y))
 obj-y += moxiesim.o