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>

scripts/dtc/pylibfdt/libfdt.i_shipped

@@ -7,6 +7,10 @@
 
 %module libfdt
 
+%begin %{
+#define PY_SSIZE_T_CLEAN
+%}
+
 %include <stdint.i>
 
 %{