buildworker: add swig package

buildworker: add swig package

The U-Boot build for more and more SoCs is using binman by default to
combine the images (SPL, U-Boot, ...). Binman is build from the U-Boot
project and it needs swig to build. We have multiple patches in OpenWrt
to remove this dependency from U-Boot, but it costs more and more time
to revert back to the old code. We have them in sunxi and rockchip
U-Boot, the Mediatek U-Boot build failed some time ago because of
missing swig in build bots.

In U-Boot 2021.01 the binman usage changed again, so it needs further
adaptations.

Building swig in OpenWrt tools will not be easy. We needs swig with
Python bindings and this version needs the Python development headers to
build.

References: https://lists.infradead.org/pipermail/openwrt-devel/2021-February/033654.html
Suggested-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Petr Štetiar <ynezz@true.cz>