libstdc++: Make net::service_already_exists default constructible

The LWG issue I created is Tentatively Ready and proposes to declare a
public default constructor, rather than the private one I added
recently.

	* include/experimental/executor (service_already_exists): Make default
	constructor public (LWG 3414).
	* testsuite/experimental/net/execution_context/make_service.cc: Check
	the service_already_exists can be default constructed.