Add a license to wasix-witx

The wasix-org/wasix-witx repository does not currently include an explicit license. Please could one be added, covering the WITX files?

This would clarify the licensing of headers and bindings generated from the WITX files in wasix-libc and wasix-abi-rust, including api_wasix.h used by guests.

(Raised here as wasix-witx has issues disabled.)