Enable RCTDevLoadingView with another flag

Reviewed By: javache

Differential Revision: D8723591

fbshipit-source-id: 196cecd73ba0e1a1d34ec9a775cf03a25f352401

Author: yancouto

React/DevSupport/RCTDevLoadingView.m

@@ -14,7 +14,7 @@
 #import "RCTModalHostViewController.h"
 #import "RCTUtils.h"
 
-#if RCT_DEV
+#if RCT_DEV | RCT_ENABLE_LOADING_VIEW
 
 static BOOL isEnabled = YES;