Move a few classes into an anonymous namespace (#4713)

src/os.cc

@@ -82,6 +82,8 @@ FMT_BEGIN_NAMESPACE
 #ifdef _WIN32
 namespace detail {
 
+namespace {
+
 class system_message {
   system_message(const system_message&) = delete;
   void operator=(const system_message&) = delete;
@@ -129,6 +131,8 @@ class utf8_system_category final : public std::error_category {
   }
 };
 
+}  // namespace
+
 }  // namespace detail
 
 FMT_API const std::error_category& system_category() noexcept {