Makefile: when building libfdt only, do not add unneeded deps

implemented originally for the QEMU consumer of libfdt.

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Message-Id: <20200512103315.1926-1-cfontana@suse.de>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>