Ignore target in crates folder (#3723)

Summary:
Rust generates some build artifacts in creates/x/target that should be ignored. I think this happens when using vscode rust extension.

Pull Request resolved: #3723

Test Plan:
Imported from GitHub, without a `Test Plan:` line.

**Static Docs Preview: relay**
|[Full Site](https://our.intern.facebook.com/intern/staticdocs/eph/D33476753/V2/relay/)|

|**Modified Pages**|

Reviewed By: kassens

Differential Revision: D33476753

Pulled By: alunyov

fbshipit-source-id: 080769642f7f82a7927cf803683a58dae32a11d3

.gitignore

@@ -2,6 +2,7 @@
 node_modules
 npm-debug.log
 /compiler/target
+/compiler/crates/*/target
 /dist/
 /lib/
 /website/build