Add void prototype to RCTAppearance.h (#47564)

Summary:
Resolve warning on ios build:
```
.../ios/Pods/Headers/Public/React-Core/React/RCTAppearance.h:16:60 A function declaration without a prototype is deprecated in all versions of C
```

## Changelog:

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[IOS] [FIXED] - Resolve deprecated function prototype warning in RCTAppearance.h

Pull Request resolved: #47564

Test Plan:
Jest Result (`yarn test`):
```
Test Suites: 234 passed, 234 total
Tests:       2 skipped, 4899 passed, 4901 total
Snapshots:   1687 passed, 1687 total
Time:        46.387 s
Ran all test suites.
```

Reviewed By: cipolleschi

Differential Revision: D65816584

Pulled By: javache

fbshipit-source-id: 212021c39dfde7e638752940e67a9f964d2194ab

packages/react-native/React/CoreModules/RCTAppearance.h

@@ -14,7 +14,7 @@
 RCT_EXTERN void RCTEnableAppearancePreference(BOOL enabled);
 RCT_EXTERN void RCTOverrideAppearancePreference(NSString *const);
 RCT_EXTERN void RCTUseKeyWindowForSystemStyle(BOOL useMainScreen);
-RCT_EXTERN NSString *RCTCurrentOverrideAppearancePreference();
+RCT_EXTERN NSString *RCTCurrentOverrideAppearancePreference(void);
 RCT_EXTERN NSString *RCTColorSchemePreference(UITraitCollection *traitCollection);
 
 @interface RCTAppearance : RCTEventEmitter <RCTBridgeModule>