Use lower-case file extension for LICENSE.txt

[silwol]

The LICENSE.txt file should be distributed to crates.io, but it wasn't due to the Cargo.toml entry in the includes field being LICENSE.txt with lower-case file extension while the file itself had an upper-case extension.

Feel free to fix the issue in the opposite direction, by renaming the Cargo.toml entry in the includes field, if that's what you prefer.

Relevant for the rejection referenced in https://github.com/rust-lang/compiler-builtins/issues/307#issuecomment-589576798