Use a consistent approach to extension availability (#269)

Author: petea

GoogleSignIn/Sources/Public/GoogleSignIn/GIDSignIn.h

@@ -160,7 +160,8 @@ typedef NS_ERROR_ENUM(kGIDSignInErrorDomain, GIDSignInErrorCode) {
                                       hint:(nullable NSString *)hint
                           additionalScopes:(nullable NSArray<NSString *> *)additionalScopes
                                 completion:(nullable void (^)(GIDUserAuth *_Nullable userAuth,
-                                                              NSError *_Nullable error))completion;
+                                                              NSError *_Nullable error))completion
+    NS_EXTENSION_UNAVAILABLE("The sign-in flow is not supported in App Extensions.");
 
 #elif TARGET_OS_OSX
 /// Starts an interactive sign-in flow on macOS.