gbm-sys: ensure license file is published

As the license file is present at the root of the repository, it isn't
part of the `gbm-sys` crate on crates.io, as it lies in a subfolder.
Ensure this file is published by instructing cargo where to find it.

Author: a-wai

gbm-sys/Cargo.toml

@@ -9,6 +9,7 @@ repository = "https://github.com/Drakulix/gbm.rs/tree/master/gbm-sys"
 keywords = ["gbm", "bindings"]
 categories = ["external-ffi-bindings"]
 license = "MIT"
+license-file = "../LICENSE"
 edition = "2021"
 
 [lib]