uboot-sifiveu: fix build with swig 4.3.0

uboot-sifiveu: fix build with swig 4.3.0

Fixes the following error by backporting upstream update:
```
scripts/dtc/pylibfdt/libfdt_wrap.c: In function ‘_wrap_fdt_next_node’:
scripts/dtc/pylibfdt/libfdt_wrap.c:5581:17: error: too few arguments to function ‘SWIG_Python_AppendOutput’
 5581 |     resultobj = SWIG_Python_AppendOutput(resultobj, val);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~
```

This is the same fix as applied in 4e68103c4eb9 ("uboot-rockchip: fix build with swig 4.3.0")

Link: https://github.com/openwrt/openwrt/pull/20538
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>