libfdt: fix build with Python 3

For some reason Python 3 seems to think it does not need to build
the library. Using the --force parameter makes sure that the library
gets built always. This is especially important since we move the
library in the next step of the Makefile, hence forcing a rebuild
every time the higher level Makefile triggers a rebuild is required
to make sure the library is always there.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Simon Glass <sjg@chromium.org>