Only use Flipper in DEBUG builds

Summary:
as discussed in https://github.com/facebook/flipper/issues/976#issuecomment-634917766,

this solves the issue that FlipperKit is accidentally included in production builds ones react-native-flipper is installed, due to it's dependency on Flipper kit

This change is safe because the bindings did already have a check to verify that the Flipper bindings are available at all, and silenty skips their absence in production builds: https://www.internalfb.com/intern/diffusion/FBS/browse/master/xplat/sonar/react-native/react-native-flipper/index.js?commit=b7dd7e05b177&lines=128

Reviewed By: passy

Differential Revision: D21749213

fbshipit-source-id: 2c4435c7a375fbc24f89159855d3e0297f3aa9a4

react-native/react-native-flipper/ios/FlipperReactNativeJavaScriptPluginManager.h

@@ -5,6 +5,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+#ifdef DEBUG
+
 #import <React/RCTBridgeModule.h>
 
 #import <FlipperKit/FlipperClient.h>
@@ -44,3 +46,5 @@ NS_ASSUME_NONNULL_BEGIN
 @end
 
 NS_ASSUME_NONNULL_END
+
+#endif