Implement requiresMainQueueSetup in RCTTVNavigationEventEmitter

Summary:
Fix the following warning:

> Module RCTTVNavigationEventEmitter requires main queue setup since it overrides `init` but doesn't implement `requiresMainQueueSetup`. In a future release React Native will default to initializing all native modules on a background thread unless explicitly opted-out of.

Trivial change.

[IOS] [MINOR] [RCTTVNavigationEventEmitter] - Implement `requiresMainQueueSetup` in `RCTTVNavigationEventEmitter`
Closes https://github.com/facebook/react-native/pull/16915

Differential Revision: D6394636

Pulled By: shergin

fbshipit-source-id: 3981655832715e73e93ef917d987e25097029b84

React/Modules/RCTTVNavigationEventEmitter.m

@@ -17,6 +17,11 @@ static NSString *const TVNavigationEventName = @"onTVNavEvent";
 
 RCT_EXPORT_MODULE()
 
++ (BOOL)requiresMainQueueSetup
+{
+  return NO;
+}
+
 - (instancetype)init
 {
   if (self = [super init]) {