Fixed compiler error on ios.
Fixed compiler error on ios.

flutter-webrtc/flutter-webrtc,flutter-webrtc/flutter-webrtc,flutter-webrtc/flutter-webrtc,flutter-webrtc/flutter-webrtc,flutter-webrtc/flutter-webrtc,flutter-webrtc/flutter-webrtc,flutter-webrtc/flutter-webrtc,flutter-webrtc/flutter-webrtc