libstdc++: Use std::add_lvalue_reference in src/c++11/mutex.cc

Adjust the return type to be consistent with how it's declared in
<mutex>.

libstdc++-v3/ChangeLog:

	* src/c++11/mutex.cc [_GLIBCXX_NO_EXTERN_THREAD_LOCAL]
	(__get_once_call): Use std::add_lvalue_reference.

libstdc++-v3/src/c++11/mutex.cc

@@ -42,7 +42,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   __get_once_callable() noexcept
   { return __once_callable; }
 
-  __typeof__(void (*)())&
+  std::add_lvalue_reference<void (*)()>::type
   __get_once_call() noexcept
   { return __once_call; }