libfdt: Fix build with python 3.10

Python 3.10 requires defining PY_SSIZE_T_CLEAN. This will be fixed in
swig 4.10 but it is not clear when it will be released. There was a
warning since python 3.8.

Link: https://github.com/swig/swig/pull/2277



Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>