fixing ATOMIC_VAR_INIT call (facebook#26238)

Summary:
We're trying to build react-native on Windows (part of the Microsoft\react-native-windows project) with MSVC compiler with WITH_FBSYSTRACE set to true (to route the traces to ETW). This change is to fix a compilation error due to the non standard usage of ATOMIC_VAR_INIT macro called with no parameters. It's not absolutely clear to me the objective of this macro in the standard at all (to be used in c context ?), and which compiler does support this parameterless version (gcc?).

Also, I'm more inclined towards changing the statement to just "std::atomic_uint_least32_t m_systraceCookie{};". Please confirm.

## Changelog

[General] [Fixed] - Removing the non-standard usage of ATOMIC_VAR_INIT macro from code with systrace enabled
Pull Request resolved: facebook#26238

Test Plan: Build verification should suffice as there is no semantic change introduced by this change.

Differential Revision: D17259213

Pulled By: cpojer

fbshipit-source-id: 9fe44f9220f18399a58f94f0f01d5fa93e6458e0

Author: mganandraj

ReactCommon/cxxreact/NativeToJsBridge.h

@@ -107,7 +107,7 @@ class NativeToJsBridge {
   bool m_applicationScriptHasFailure = false;
 
   #ifdef WITH_FBSYSTRACE
-  std::atomic_uint_least32_t m_systraceCookie = ATOMIC_VAR_INIT();
+  std::atomic_uint_least32_t m_systraceCookie = ATOMIC_VAR_INIT(0);
   #endif
 };