Change forward declaration for SignInResult to struct

SignInResult is declared as a struct in user.h but forward declared here as a class.
This did not cause error in our build but is causing compiler error for user.

Fixed #120

PiperOrigin-RevId: 329958702

Author: chkuang-g

auth/src/include/firebase/auth.h

@@ -47,7 +47,7 @@ class PhoneAuthProvider;
 struct AuthCompletionHandle;
 class FederatedAuthProvider;
 class FederatedOAuthProvider;
-class SignInResult;
+struct SignInResult;
 
 /// @brief Firebase authentication object.
 ///