Fix missing `atomic` import (#37621)

Summary:
Native error: `Declaration of 'atomic_bool' must be imported from module 'std.atomic' before it is required.`

## Changelog:

[IOS] [FIXED] - Add missing `atomic` import

Pull Request resolved: https://github.com/facebook/react-native/pull/37621

Test Plan: This build error appears to only occur when other build errors occur, but it supersedes the other build errors, increasing the difficulty of debugging the root build error(s).

Reviewed By: NickGerleman

Differential Revision: D46284208

Pulled By: sammy-SC

fbshipit-source-id: b36cb3a9166b1e7cef0121a2e635a191bd7c3a5e

packages/react-native/React/CxxBridge/RCTMessageThread.h

@@ -12,6 +12,7 @@
 
 #import <React/RCTJavaScriptExecutor.h>
 #import <cxxreact/MessageQueueThread.h>
+#import <atomic>
 
 namespace facebook::react {