Fix Xcode 11 build (#25146)

Summary:
Fixes build in Xcode 11 beta, the signature for `__unused` was changed. This adds a new check for the new style.

## Changelog

[iOS] [Fixed] - Xcode 11 beta build
Pull Request resolved: https://github.com/facebook/react-native/pull/25146

Differential Revision: D15628404

Pulled By: cpojer

fbshipit-source-id: 781a188a0e1562a3316fbe62920b12b03a44e4a7

React/Base/RCTModuleMethod.mm

@@ -91,6 +91,7 @@ static BOOL RCTParseSelectorPart(const char **input, NSMutableString *selector)
 static BOOL RCTParseUnused(const char **input)
 {
   return RCTReadString(input, "__attribute__((unused))") ||
+         RCTReadString(input, "__attribute__((__unused__))") ||
          RCTReadString(input, "__unused");
 }