Fix Undefined symbols issue

[alvarhansen]

Add AppAuthCore as explicit dependency to GoogleSignIn target as there are symbols that it uses from that library.

It is needed where GoogleSignIn is used as dynamic library/framework. This is the case when GoogleSignIn is added to multiple targets in dependency graph where 2 dynamic libraries/frameworks exist.

Example of it can be seen in project that @cjwirth provided in #408.

Fixes: https://github.com/google/GoogleSignIn-iOS/issues/408