Ignore cxx folders for all of flutter/flutter (#160381)

Related to flutter/flutter#160372


## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

Author: reidbaker

.gitignore

@@ -74,6 +74,7 @@ unlinked_spec.ds
 **/android/**/GeneratedPluginRegistrant.java
 **/android/key.properties
 *.jks
+**/.cxx/
 
 # iOS/XCode related
 **/ios/**/*.mode1v3