Wrap private module fragment content within conditional extern "C++", to match declarations. (#4083)

src/fmt.cc

@@ -127,9 +127,17 @@ extern "C++" {
 module :private;
 #endif
 
+#ifdef FMT_ATTACH_TO_GLOBAL_MODULE
+extern "C++" {
+#endif
+
 #if FMT_HAS_INCLUDE("format.cc")
 #  include "format.cc"
 #endif
 #if FMT_OS && FMT_HAS_INCLUDE("os.cc")
 #  include "os.cc"
 #endif
+
+#ifdef FMT_ATTACH_TO_GLOBAL_MODULE
+}
+#endif