Add missing override annotation in GoogleSignInPlugin (flutter#155)

Author: tvolkert

packages/google_sign_in/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java

@@ -391,6 +391,7 @@ public void onConnectionFailed(@NonNull final ConnectionResult result) {
               result.getErrorCode(),
               REQUEST_CODE_RESOLVE_ERROR,
               new DialogInterface.OnCancelListener() {
+                @Override
                 public void onCancel(DialogInterface dialog) {
                   if (pendingOperation != null && pendingOperation.method.equals(METHOD_INIT)) {
                     finishWithError(