Fix RNTester build

Summary: WKWebViews aren't available on Apple TV. When I landed [the WKWebView stack](https://our.intern.facebook.com/intern/diff/D9362001/), I inadvertently broke the RNTester osTV build. This diff fixes it.

Reviewed By: shergin

Differential Revision: D9844322

fbshipit-source-id: 6de0fbfd13aba38cca7530c9fb486c7cde0afc82

React/Base/RCTConvert.h

@@ -15,7 +15,9 @@
 #import <React/RCTPointerEvents.h>
 #import <React/RCTTextDecorationLineType.h>
 #import <yoga/Yoga.h>
+#if TARGET_OS_IPHONE && WEBKIT_IOS_10_APIS_AVAILABLE
 #import <WebKit/WebKit.h>
+#endif
 
 /**
  * This class provides a collection of conversion functions for mapping