crates published to crates.io do not contain the required MIT license file

The libspa-sys, libspa, pipewire-sys, and pipewire crates as published to crates.io all do not contain a license file. To comply with the terms of the MIT license, including a LICENSE file with redistributed sources is a requirement. This is also a problem for packaging these crates for Fedora, for example. Adding LICENSE -> ../LICENSE symlinks inside the four folders should fix the problem, at least for the next versions that are published.