fdt: Fix compile error for !OFNODE_MULTI_TREE

Required to fix the following compile error when building sandbox:
/tmp/cci9ibby.ltrans21.ltrans.o: In function `do_cedit_load':
<artificial>:(.text+0x601d): undefined reference to `oftree_dispose'

Signed-off-by: Sean Edmond <seanedmond@microsoft.com>
Reviewed-by: Simon Glass <sjg@chromium.org>