libselinux: fix build

Building libselinux 2.9 with python 3.8 causes several linker failures[1].
As this breaks several packages on staging and the glibc-230 integration
jobset[2], this should be fine as workaround.

This change can be reverted when updating to libselinux 3.0, ArchLinux
already has `libselinux` 3.0 with python 3.8[3].

[1] https://hydra.nixos.org/build/109960000
[2] https://hydra.nixos.org/eval/1564244
[3] https://aur.archlinux.org/cgit/aur.git/commit/?h=libselinux&id=f717bef8131622ad8545f4bdffe7bafdc35e9808