Setup fails if pkg-config is not installed

[TheStaticTurtle]

Picosetup fails to build picotool (and maybe other things) if pkg-config is not installed. It causes libusb to not be found.
Fix is just to apt install it:

Environment: 64bit Ubuntu 20.04 in a lxc container

[aallan]

Environment: 64bit Ubuntu 20.04 in a lxc container

The script was written assuming that it's being run on a Raspberry Pi and the latest version of Raspberry Pi OS, running on other platforms it warns you that "Not running on a Raspberry Pi. Use at your own risk!"

We don't intend to support other platforms at this time.