Fix OSS BUCK codegen (#33197)

Summary:
This fixes a mistake I made introduced in D34451208 (a0a2958cda) where the OSS buck file was missing the `rn_codegen_cxx_modules` import.

Changelog:
Internal

Pull Request resolved: https://github.com/facebook/react-native/pull/33197

Reviewed By: JoshuaGross

Differential Revision: D34541508

fbshipit-source-id: d99472b69aed78897b88acabee4a7faa63c7bb72

tools/build_defs/oss/rn_codegen_defs.bzl

@@ -9,6 +9,7 @@
 load(
     "//packages/react-native-codegen:DEFS.bzl",
     _rn_codegen_components = "rn_codegen_components",
+    _rn_codegen_cxx_modules = "rn_codegen_cxx_modules",
     _rn_codegen_modules = "rn_codegen_modules",
 )
 load("//tools/build_defs:fb_native_wrapper.bzl", "fb_native")
@@ -18,6 +19,7 @@ load(
 )
 
 rn_codegen_components = _rn_codegen_components
+rn_codegen_cxx_modules = _rn_codegen_cxx_modules
 rn_codegen_modules = _rn_codegen_modules
 
 def rn_codegen(