fix: flowlog has no default child (#21045)

Calling defaultChild on `FlowLog` results in undefined. Explicitly set CfnResource as default child

----

### All Submissions:

* [ ] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-ec2/lib/vpc-flow-logs.ts

@@ -510,5 +510,6 @@ export class FlowLog extends FlowLogBase {
     });
 
     this.flowLogId = flowLog.ref;
+    this.node.defaultChild = flowLog;
   }
 }

packages/@aws-cdk/aws-ec2/test/vpc-flow-logs.test.ts

@@ -442,6 +442,16 @@ describe('vpc flow logs', () => {
       },
     });
   });
+  test('flowlog has defaultchild', () => {
+    const stack = new Stack();
+    const vpc = new Vpc(stack, 'VPC');
+
+    const flowlog = new FlowLog(stack, 'FlowLog', {
+      resourceType: FlowLogResourceType.fromVpc(vpc),
+    });
+
+    expect(flowlog.node.defaultChild).toBeDefined();
+  });
 });
 
 function getTestStack(): Stack {