Declare newsfeed as an optional dependency of SIEM

We're going to use the availability of the newsfeed plugin as part of our
determination for whether or not to show the security newsfeed. If users
set `newsfeed.enabled: false`, the plugin will be unavailable and the
security feed will not be shown.

x-pack/legacy/plugins/siem/public/plugin.tsx

@@ -14,6 +14,7 @@ import {
 import { HomePublicPluginSetup } from '../../../../../src/plugins/home/public';
 import { DataPublicPluginStart } from '../../../../../src/plugins/data/public';
 import { IEmbeddableStart } from '../../../../../src/plugins/embeddable/public';
+import { Start as NewsfeedStart } from '../../../../../src/plugins/newsfeed/public';
 import { Start as InspectorStart } from '../../../../../src/plugins/inspector/public';
 import { IUiActionsStart } from '../../../../../src/plugins/ui_actions/public';
 import { UsageCollectionSetup } from '../../../../../src/plugins/usage_collection/public';
@@ -29,7 +30,7 @@ export interface StartPlugins {
   data: DataPublicPluginStart;
   embeddable: IEmbeddableStart;
   inspector: InspectorStart;
-  newsfeed?: unknown;
+  newsfeed?: NewsfeedStart;
   uiActions: IUiActionsStart;
 }
 export type StartServices = CoreStart & StartPlugins;