Use explicit cast in plugin.tsx

Summary:
Marginally safer than ignoring the entire line, which
makes type-checking weaker on subsequent references.

Reviewed By: jknoxville

Differential Revision: D17736479

fbshipit-source-id: 13afcb5f08e53016b7a6cdc7ea135e726586a67b

src/plugin.tsx

@@ -209,8 +209,7 @@ export class FlipperPlugin<
     // @ts-ignore constructor should be assigned already
     const {id} = this.constructor;
     this.subscriptions = [];
-    // @ts-ignore props.target will be instance of Client
-    this.realClient = props.target;
+    this.realClient = props.target as Client;
     this.client = {
       call: (method, params) => this.realClient.call(id, method, true, params),
       send: (method, params) => this.realClient.send(id, method, params),