Update ignore file to include ReactBuildConfig in the npm package (#…

…45279)

Summary:
Changes `.npmignore` file to only exclude the `ReactAndroid/build` directory instead of all `build` directories under `ReactAndroid` (which included the `ReactAndroid/src/main/java/com/facebook/react/common/build` package). This problem was caused by the newer version of NPM being used.

Closes #45204

## Changelog:

[ANDROID] [FIXED] - Fixed build from source failing due to a missing file

Pull Request resolved: #45279

Test Plan:
Run `npm pack` or `npm publish -dry-run`.

Before this change it includes 3774 files in the package and `ReactBuildConfig` isn't included. After this change it includes 3775 files in the package and `ReactBuildConfig` is included.

Reviewed By: javache

Differential Revision: D59371555

Pulled By: cortinico

fbshipit-source-id: f54f1e88e30429d538b9e160e6ce20d994c5d1b8

packages/react-native/ReactAndroid/.npmignore

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