Do not bundle ReactAndroid/.cxx inside the npm package

Summary:
With CMake we're accidentally bundling the .cxx folder inside the NPM package. This is making the package explode and is letting the publishing of nightly fail

Changelog:
[Internal] [Changed] - Do not bundle ReactAndroid/.cxx inside the npm package

Reviewed By: ShikaSD

Differential Revision: D35110743

fbshipit-source-id: fb51f0c4948479e2fcf96f5ac8df6362d5875421

Author: cortinico

ReactAndroid/.npmignore

@@ -1,5 +1,7 @@
-# Make sure we never publish ReactAndroid/build (Gradle output) to npm.
-# The folder is huge (> 100MB)!
+# Make sure we never publish ReactAndroid/build (Gradle output)
+# or ReactAndroid/.cxx (CMake output) to npm.
+# Those folders are huge (> 100MB)!
 build/
+.cxx/
 # Exclude buck config/jars for third-party libraries
 src/main/third-party/