add libc fixup (#40)

Summary:
fixes warning
```
[WARN  reindeer::fixups] libc-0.2.153 has a build script, but I don't know what to do with it: Unresolved build script at ../../../../.cargo/registry/src/index.crates.io-6f17d22bba15001f/libc-0.2.153/build.rs.
```

Pull Request resolved: #40

Test Plan: https://github.com/shayne-fletcher/reindeer/actions/runs/7764158226

Reviewed By: dtolnay

Differential Revision: D53377343

Pulled By: shayne-fletcher

fbshipit-source-id: 2459ce93b95b77800afade7293381c8be9326911

example/third-party/fixups/libc/fixups.toml

@@ -0,0 +1,2 @@
+[[buildscript]]
+[buildscript.rustc_flags]