Fix building example project for iOS

[kevin49999]

When trying to build viam_example_app for iOS you will encounter this error:

/bin/sh: /packages/flutter_tools/bin/xcode_backend.sh: No such file or directory
Command PhaseScriptExecution failed with a nonzero exit code

That's because the failing script references FLUTTER_ROOT which is undefined

The Generated.xcconfig file defines FLUTTER_ROOT as well as several other properties. Including this in the Debug.xcconfig resolves the error

Encountered this error while pairing w/ @benjirewis when he was trying to run the example

[njooma]

I believe this is a generated file, so not sure how it would've gotten out of sync