register the live activities module

* Fixes this error when the live activity methods were called:
- Unhandled Exception: MissingPluginException(No implementation found for method OneSignal#enterLiveActivity on channel OneSignal#liveactivities)

Author: nan-li

ios/Classes/OneSignalPlugin.m

@@ -33,6 +33,7 @@
 #import "OSFlutterSession.h"
 #import "OSFlutterLocation.h"
 #import "OSFlutterInAppMessages.h"
+#import "OSFlutterLiveActivities.h"
 
 
 @interface OneSignalPlugin ()
@@ -70,7 +71,7 @@ + (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {
     [OSFlutterSession registerWithRegistrar:registrar];
     [OSFlutterLocation registerWithRegistrar:registrar];
     [OSFlutterInAppMessages registerWithRegistrar:registrar];
-    
+    [OSFlutterLiveActivities registerWithRegistrar:registrar];
 }
 
 - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {