fix(firebase_crashlytics): Add Flutter dependency to podspec

[jmagman]

Description

flutter/flutter#78592 stopped trying Flutter against pods that don't explicitly declare Flutter as a dependency (to fix a bitcode linking issue). This means that all Flutter plugins rely on correct podspec dependencies set up, so either s.dependency 'Flutter' or s.dependency 'FlutterMacOS'.

I don't see any other podspecs that are missing it.

Related Issues

Fixes #5440

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See [Contributor Guide]).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the [Flutter Style Guide].
• I signed the [CLA].
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

[Salakar]

Thank you