Podspec: add required "DevSupport" dependency "InspectorInterfaces.{cpp,h}

Summary:
This PR fixes #14975.
* Podspec: add InspectorInterfaces.h/cpp to the "jschelpers_legacy" subspec.

1. lint the Podspecs: `./scripts/process-podspecs.sh`
2. check that an iOS project can integrate "DevSupport" via Cocoapods again. I'll already checked it on this branch of the demo project: https://github.com/HeEAaD/Demo-ReactNative-0.47.0-rc2-Podspec-issue/tree/fix
Closes https://github.com/facebook/react-native/pull/14988

Differential Revision: D5411733

Pulled By: javache

fbshipit-source-id: bb46cf3461e0857c7508264dafa204067af182c6