Fix missing LICENSE file in published ffms2-sys crate

[musicinmybrain]

The chosen MIT license requires the copyright and permission notices (license text) to be “included in all copies or substantial portions of the Software.”

Before this PR:

$ git clone https://github.com/rust-av/ffms2-rs.git
$ cd ffms2-rs/ffms2-sys/
$ cargo package
$ ls ../target/package/ffms2-sys-0.3.0
build.rs  Cargo.lock  Cargo.toml  Cargo.toml.orig  data  README.md  src

After this PR:

$ cargo package
$ ls ../target/package/ffms2-sys-0.3.0
build.rs  Cargo.lock  Cargo.toml  Cargo.toml.orig  data  LICENSE  README.md  src