Add RN_BUILDING to fantom (#58392)

Summary:
Pull Request resolved: #58392

Update Fantom build config to compile with `RN_BUILDING` defined, allowing all includes within Fantom.

Changelog: [Internal]

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Differential Revision: D117860963

fbshipit-source-id: d83eb7e3d6d3d8e45efd3b1b49e68894c1c63b09

Author: j-piasecki

private/react-native-fantom/tester/CMakeLists.txt

@@ -38,6 +38,10 @@ add_fantom_third_party_subdir(folly)
 add_fantom_third_party_subdir(gflags)
 add_fantom_third_party_subdir(nlohmann_json)
 
+# Marks everything added below as React Native's own build, to enable private
+# includes in Fantom.
+add_compile_definitions(RN_BUILDING)
+
 add_subdirectory(${FANTOM_CODEGEN_DIR} codegen)
 
 add_library(glog_init INTERFACE)