fix(iOS): make RCTMountingTransactionObserving implementation new-arch only (#2624)

## Description

I've made mistake in #2466 - the aforementioned protocol does not exist
on Paper.

## Changes

Use ifdef directive to prevent build errors.


## Test code and steps to reproduce

iOS + old-arch should just build.

## Checklist

- [ ] Ensured that CI passes

Author: kkafar

ios/RNSScreenStackHeaderConfig.mm

@@ -59,8 +59,10 @@ + (BOOL)RNSisBlank:(NSString *)string
 
 @end
 
+#ifdef RCT_NEW_ARCH_ENABLED
 @interface RNSScreenStackHeaderConfig () <RCTMountingTransactionObserving>
 @end
+#endif // RCT_NEW_ARCH_ENABLED
 
 @implementation RNSScreenStackHeaderConfig {
   NSMutableArray<RNSScreenStackHeaderSubview *> *_reactSubviews;