Support Ubuntu 20.04 LTS (Focal Fossa)

[smhanov]

This is confirmation that following the instructions on https://github.com/AdnanHodzic/displaylink-debian works with Ubuntu 20.04.

With the official drivers, even after installing flex and bison, it will not work because the makefile cannot find Build.include in the kernel headers.

After running:
sudo apt install git
git clone https://github.com/AdnanHodzic/displaylink-debian.git
cd displaylink-debian/ && sudo ./displaylink-debian.sh

Then, reboot, and "delete all secure boot variables" in bios or otherwise remove secureboot.