device-tree: Make a common-obj

There is no reason for device tree API to be built per-target.
common-obj it. There is an extraneous inclusion of config.h that
needs to be removed.

Cc: Alexander Graf <agraf@suse.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

Makefile.objs

@@ -76,6 +76,8 @@ common-obj-$(CONFIG_SECCOMP) += qemu-seccomp.o
 
 common-obj-$(CONFIG_SMARTCARD_NSS) += $(libcacard-y)
 
+common-obj-$(CONFIG_FDT) += device_tree.o
+
 ######################################################################
 # qapi