libstdc++: Fix errors when syncbuf is used without RTTI

libstdc++-v3/ChangeLog:

	* include/std/ostream (__syncbuf_base::_S_get): Mark parameter
	as unused and only use dynamic_cast when RTTI is enabled.

libstdc++-v3/include/std/ostream

@@ -783,10 +783,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
     public:
       static bool*
-      _S_get(basic_streambuf<_CharT, _Traits>* __buf) noexcept
+      _S_get(basic_streambuf<_CharT, _Traits>* __buf [[maybe_unused]]) noexcept
       {
+#if __cpp_rtti
 	if (auto __p = dynamic_cast<__syncbuf_base*>(__buf))
 	  return &__p->_M_emit_on_sync;
+#endif
 	return nullptr;
       }