Explicit cast instead of ts-ignore

Summary: Slightly safer.

Reviewed By: jknoxville

Differential Revision: D17686947

fbshipit-source-id: 2c478707e20a1475427be4a652283960e4515cf9

src/plugin.tsx

@@ -180,8 +180,7 @@ export class FlipperDevicePlugin<
 
   constructor(props: Props<P>) {
     super(props);
-    // @ts-ignore props.target will be instance of Device
-    this.device = props.target;
+    this.device = props.target as BaseDevice;
   }
 
   _init() {