OpenWrt add dependency to swig for U-Boot builds

[Petr Štetiar]

Hi,

> Hauke Mehrtens <hauke at hauke-m.de> [2021-01-31 19:43:42]:
> 
> > Does anyone have an opinion on this topic?
> 
> I don't see other option ATM.

so I went ahead and added swig[1] to the buildworker image.

1. https://git.openwrt.org/3776a9157532e69478ef03236d22254267a4f12e

Cheers,

Petr