fix(vendor): declare build.rs as included file in Cargo.toml (#2204)

Author: ad0

Cargo.toml

@@ -8,7 +8,7 @@ authors     = ["The nix-rust Project Developers"]
 repository  = "https://github.com/nix-rust/nix"
 license     = "MIT"
 categories  = ["os::unix-apis"]
-include = ["src/**/*", "test/**/*", "LICENSE", "README.md", "CHANGELOG.md"]
+include = ["build.rs", "src/**/*", "test/**/*", "LICENSE", "README.md", "CHANGELOG.md"]
 
 [package.metadata.docs.rs]
 all-features = true