Making JSI_EXPORT macro definition conditional (#22561)

Summary:
This change enables defining the macro at a more global and central location. And thereby allowing us to building this file using MSVC.
Pull Request resolved: https://github.com/facebook/react-native/pull/22561

Reviewed By: fkgozali

Differential Revision: D13529762

Pulled By: RSNara

fbshipit-source-id: c78abb26f47aba25d50305c5bc0b23b6568f1243

ReactCommon/jsi/jsi.h

@@ -13,7 +13,9 @@
 #include <string>
 #include <vector>
 
+#ifndef JSI_EXPORT
 #define JSI_EXPORT __attribute__((visibility("default")))
+#endif
 
 class FBJSRuntime;
 namespace facebook {