title: Firebase / Google VR linker error in Unity - iOS
body: <p>I'm building an iOS app using unity (v2019.1.0f2) and firebase (v6.0) and Google VR (v1.200). When I use an empty project and add firebase, it would build successfully. If I clicked the Virtual Reality support, it import stuff then when I build. I face the following problem in Xcode</p>
  <pre><code>Undefined symbol: _OBJC_CLASS_$_FIRPhoneAuthCredential
  Undefined symbol: _OBJC_CLASS_$_FIRGameCenterAuthProvider
  Undefined symbol: _OBJC_CLASS_$_GKLocalPlayer
  Undefined symbol: _OBJC_CLASS_$_FIRPhoneAuthProvider
  Undefined symbol: _OBJC_CLASS_$_FIROAuthProvider
  Undefined symbol: _OBJC_CLASS_$_FIRFacebookAuthProvider
  Undefined symbol: _OBJC_CLASS_$_FIREmailAuthProvider
  Undefined symbol: _OBJC_CLASS_$_FIRGoogleAuthProvider
  Undefined symbol: _OBJC_CLASS_$_FIRGitHubAuthProvider
  Undefined symbol: _OBJC_CLASS_$_FIRTwitterAuthProvider
  Undefined symbol: _OBJC_CLASS_$_FIRAuth
  </code></pre>
  <p>Any idea why?</p>