fix(aws-ecs): Add type annotation for the workaround to missing import in d.ts file

[cockscomb]

The @aws-cdk/aws-ecs/lib/fargate/fargate-task-definition.d.ts file was emitted wrong because TypeScript have an issue (microsoft/TypeScript#26969). So I think we have to workaround by adding a type annotation.

Before

(Comments removed)

import cdk = require('@aws-cdk/cdk');
import { CommonTaskDefinitionProps, TaskDefinition } from '../base/task-definition';

export interface FargateTaskDefinitionProps extends CommonTaskDefinitionProps {
    cpu?: string;
    memoryMiB?: string;
}

export declare class FargateTaskDefinition extends TaskDefinition {
    readonly networkMode: NetworkMode;
    constructor(parent: cdk.Construct, name: string, props?: FargateTaskDefinitionProps);
}

This emit an error like ../../node_modules/@aws-cdk/aws-ecs/lib/fargate/fargate-task-definition.d.ts(46,28): error TS2304: Cannot find name 'NetworkMode'.

After

import cdk = require('@aws-cdk/cdk');
import { CommonTaskDefinitionProps, NetworkMode, TaskDefinition } from '../base/task-definition';

export interface FargateTaskDefinitionProps extends CommonTaskDefinitionProps {
    cpu?: string;
    memoryMiB?: string;
}

export declare class FargateTaskDefinition extends TaskDefinition {
    readonly networkMode: NetworkMode;
    constructor(parent: cdk.Construct, name: string, props?: FargateTaskDefinitionProps);
}

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

[rix0rrr]

Yup. Thanks.

I'm going to add more comments inline but this is the gist of it.